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>
diff --git a/fs/nfs/namespace.c b/fs/nfs/namespace.c
index 66df08d..d398775 100644
--- a/fs/nfs/namespace.c
+++ b/fs/nfs/namespace.c
@@ -189,7 +189,7 @@
 					   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: