Reconstruct SELinux mount option

When reconstructing volumes from disk after kubelet restart, reconstruct
also context=XYZ mount option and add it to the ActualStateOfWorld.

pkg/volume/util/hostutil/hostutil.go

@@ -68,6 +68,9 @@ type HostUtils interface {
 	GetSELinuxSupport(pathname string) (bool, error)
 	// GetMode returns permissions of the path.
 	GetMode(pathname string) (os.FileMode, error)
+	// GetSELinuxMountContext returns value of -o context=XYZ mount option on
+	// given mount point.
+	GetSELinuxMountContext(pathname string) (string, error)
 }
 
 // Compile-time check to ensure all HostUtil implementations satisfy