ensure pkg/util/mount compiles & crosses

- move compile time check from linux code to generic code

pkg/util/mount/mount.go

@@ -51,6 +51,10 @@ type Interface interface {
 	GetDeviceNameFromMount(mountPath, pluginDir string) (string, error)
 }
 
+// Compile-time check to ensure all Mounter implementations satisfy
+// the mount interface
+var _ Interface = &Mounter{}
+
 // This represents a single line in /proc/mounts or /etc/fstab.
 type MountPoint struct {
 	Device string

pkg/util/mount/mount_linux.go

@@ -54,8 +54,6 @@ const (
 // kubelet is running in the host's root mount namespace.
 type Mounter struct{}
 
-var _ = Interface(&Mounter{})
-
 // Mount mounts source to target as fstype with given options. 'source' and 'fstype' must
 // be an emtpy string in case it's not required, e.g. for remount, or for auto filesystem
 // type, where kernel handles fs type for you. The mount 'options' is a list of options,