Add tmpfs detection to mounttest and unit tests

Refactor fsType() to use a platform-specific formatFsType() helper that
translates filesystem magic numbers to human-readable names. On Linux,
tmpfs filesystems now display as "tmpfs" instead of the raw magic number.

Signed-off-by: Davanum Srinivas <davanum@gmail.com>

test/images/agnhost/mounttest/mt_utils.go

@@ -39,7 +39,7 @@ func fsType(path string) error {
 		return err
 	}
 
-	fmt.Printf("mount type of %q: %v\n", path, buf.Type)
+	fmt.Printf("mount type of %q: %s\n", path, formatFsType(int64(buf.Type)))
 
 	return nil
 }

test/images/agnhost/mounttest/mt_utils_linux.go

@@ -0,0 +1,34 @@
+//go:build linux
+
+/*
+Copyright The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package mounttest
+
+import (
+	"strconv"
+
+	"golang.org/x/sys/unix"
+)
+
+// formatFsType returns a string representation of the filesystem type.
+// It translates known filesystem magic numbers to human-readable names.
+func formatFsType(fsType int64) string {
+	if fsType == unix.TMPFS_MAGIC {
+		return "tmpfs"
+	}
+	return strconv.FormatInt(fsType, 10)
+}

test/images/agnhost/mounttest/mt_utils_other.go

@@ -0,0 +1,26 @@
+//go:build !linux && !windows
+
+/*
+Copyright The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package mounttest
+
+import "strconv"
+
+// formatFsType returns a string representation of the filesystem type.
+func formatFsType(fsType int64) string {
+	return strconv.FormatInt(fsType, 10)
+}

test/images/agnhost/mounttest/mt_utils_test.go

@@ -0,0 +1,117 @@
+//go:build linux
+
+/*
+Copyright The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package mounttest
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"os"
+	"strings"
+	"testing"
+
+	"golang.org/x/sys/unix"
+)
+
+func captureStdout(f func()) string {
+	old := os.Stdout
+	r, w, _ := os.Pipe()
+	os.Stdout = w
+
+	f()
+
+	w.Close()
+	os.Stdout = old
+
+	var buf bytes.Buffer
+	io.Copy(&buf, r)
+	return buf.String()
+}
+
+func TestFsType_EmptyPath(t *testing.T) {
+	output := captureStdout(func() {
+		err := fsType("")
+		if err != nil {
+			t.Errorf("fsType(\"\") returned error: %v", err)
+		}
+	})
+
+	if output != "" {
+		t.Errorf("fsType(\"\") should not print anything, got: %q", output)
+	}
+}
+
+func TestFsType_ValidPath(t *testing.T) {
+	// Use a path that exists on the system
+	testPath := os.TempDir()
+
+	output := captureStdout(func() {
+		err := fsType(testPath)
+		if err != nil {
+			t.Errorf("fsType(%q) returned error: %v", testPath, err)
+		}
+	})
+
+	// Verify the output format
+	expectedPrefix := fmt.Sprintf("mount type of %q:", testPath)
+	if !strings.HasPrefix(output, expectedPrefix) {
+		t.Errorf("fsType(%q) output should start with %q, got: %q", testPath, expectedPrefix, output)
+	}
+
+	// Verify the output is printed exactly once (not duplicated)
+	lines := strings.Split(strings.TrimSpace(output), "\n")
+	if len(lines) != 1 {
+		t.Errorf("fsType(%q) should print exactly one line, got %d lines: %q", testPath, len(lines), output)
+	}
+}
+
+func TestFsType_NonExistentPath(t *testing.T) {
+	testPath := "/nonexistent/path/that/does/not/exist"
+
+	output := captureStdout(func() {
+		err := fsType(testPath)
+		if err == nil {
+			t.Errorf("fsType(%q) should return error for non-existent path", testPath)
+		}
+	})
+
+	// Should print an error message
+	if !strings.Contains(output, "error from statfs") {
+		t.Errorf("fsType(%q) should print error message, got: %q", testPath, output)
+	}
+}
+
+func TestFormatFsType_Tmpfs(t *testing.T) {
+	result := formatFsType(unix.TMPFS_MAGIC)
+
+	expected := "tmpfs"
+	if result != expected {
+		t.Errorf("formatFsType(TMPFS_MAGIC) = %q, want %q", result, expected)
+	}
+}
+
+func TestFormatFsType_OtherType(t *testing.T) {
+	otherType := int64(0x12345678)
+	result := formatFsType(otherType)
+
+	expected := "305419896"
+	if result != expected {
+		t.Errorf("formatFsType(%v) = %q, want %q", otherType, result, expected)
+	}
+}