Revert "fix mismerge with Trond's stuff (create_mnt_ns() export is gone now)"

This reverts commit e9496ff46a. Quoth Al:

 "it's dependent on a lot of other stuff not currently in mainline
  and badly broken with current fs/namespace.c.  Sorry, badly
  out-of-order cherry-pick from old queue.

  PS: there's a large pending series reworking the refcounting and
  lifetime rules for vfsmounts that will, among other things, allow to
  rip a subtree away _without_ dissolving connections in it, to be
  garbage-collected when all active references are gone.  It's
  considerably saner wrt "is the subtree busy" logics, but it's nowhere
  near being ready for merge at the moment; this changeset is one of the
  things becoming possible with that sucker, but it certainly shouldn't
  have been picked during this cycle.  My apologies..."

Noticed-by: Eric Paris <eparis@redhat.com>
Requested-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/nfs/super.c

@@ -2648,13 +2648,21 @@ out_freepage:
 static int nfs_follow_remote_path(struct vfsmount *root_mnt,
 		const char *export_path, struct vfsmount *mnt_target)
 {
+	struct mnt_namespace *ns_private;
 	struct nameidata nd;
 	struct super_block *s;
 	int ret;
 
+	ns_private = create_mnt_ns(root_mnt);
+	ret = PTR_ERR(ns_private);
+	if (IS_ERR(ns_private))
+		goto out_mntput;
+
 	ret = vfs_path_lookup(root_mnt->mnt_root, root_mnt,
 			export_path, LOOKUP_FOLLOW, &nd);
 
+	put_mnt_ns(ns_private);
+
 	if (ret != 0)
 		goto out_err;