Revert vendor directory to support old golang (GOPATH mode)

gomodule is still in progress to migrate for now, hence multus
team decide to keep vendor directory to support build without
gomodule.

vendor/github.com/vishvananda/netns/netns_unspecified.go

@@ -0,0 +1,43 @@
+// +build !linux
+
+package netns
+
+import (
+	"errors"
+)
+
+var (
+	ErrNotImplemented = errors.New("not implemented")
+)
+
+func Set(ns NsHandle) (err error) {
+	return ErrNotImplemented
+}
+
+func New() (ns NsHandle, err error) {
+	return -1, ErrNotImplemented
+}
+
+func Get() (NsHandle, error) {
+	return -1, ErrNotImplemented
+}
+
+func GetFromPath(path string) (NsHandle, error) {
+	return -1, ErrNotImplemented
+}
+
+func GetFromName(name string) (NsHandle, error) {
+	return -1, ErrNotImplemented
+}
+
+func GetFromPid(pid int) (NsHandle, error) {
+	return -1, ErrNotImplemented
+}
+
+func GetFromThread(pid, tid int) (NsHandle, error) {
+	return -1, ErrNotImplemented
+}
+
+func GetFromDocker(id string) (NsHandle, error) {
+	return -1, ErrNotImplemented
+}