Update vendor

vendor/github.com/moby/sys/mount/unmount_unix.go

@@ -0,0 +1,22 @@
+// +build !windows
+
+package mount
+
+import "golang.org/x/sys/unix"
+
+func unmount(target string, flags int) error {
+	err := unix.Unmount(target, flags)
+	if err == nil || err == unix.EINVAL {
+		// Ignore "not mounted" error here. Note the same error
+		// can be returned if flags are invalid, so this code
+		// assumes that the flags value is always correct.
+		return nil
+	}
+
+	return &mountError{
+		op:     "umount",
+		target: target,
+		flags:  uintptr(flags),
+		err:    err,
+	}
+}