vfs: introduce clone_private_mount()

Overlayfs needs a private clone of the mount, so create a function for
this and export to modules.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
diff --git a/include/linux/mount.h b/include/linux/mount.h
index 9262e4b..c2c561d 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -81,6 +81,9 @@
 extern struct vfsmount *mnt_clone_internal(struct path *path);
 extern int __mnt_is_readonly(struct vfsmount *mnt);
 
+struct path;
+extern struct vfsmount *clone_private_mount(struct path *path);
+
 struct file_system_type;
 extern struct vfsmount *vfs_kern_mount(struct file_system_type *type,
 				      int flags, const char *name,