nfs: ERR_PTR is expected on failure from nfs_do_clone_mount

Replace NULL with ERR_PTR(-EINVAL).
Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/namespace.c

@@ -189,7 +189,7 @@ static struct vfsmount *nfs_do_clone_mount(struct nfs_server *server,
 					   struct nfs_clone_mount *mountdata)
 {
 #ifdef CONFIG_NFS_V4
-	struct vfsmount *mnt = NULL;
+	struct vfsmount *mnt = ERR_PTR(-EINVAL);
 	switch (server->nfs_client->rpc_ops->version) {
 		case 2:
 		case 3: