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

This reverts commit e9496ff46a20a8592fdc7bdaaf41b45eb808d310. 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 <[e-mail]>
Requested-by: Al Viro <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/namespace.c

···
  * create_mnt_ns - creates a private namespace and adds a root filesystem
  * @mnt: pointer to the new root filesystem mountpoint
  */
-static struct mnt_namespace *create_mnt_ns(struct vfsmount *mnt)
+struct mnt_namespace *create_mnt_ns(struct vfsmount *mnt)
 {
 	struct mnt_namespace *new_ns;
 
···
 	}
 	return new_ns;
 }
+EXPORT_SYMBOL(create_mnt_ns);
 
 SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,
 		char __user *, type, unsigned long, flags, void __user *, data)

fs/nfs/super.c

···
 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;

include/linux/mnt_namespace.h

···
 
 struct fs_struct;
 
+extern struct mnt_namespace *create_mnt_ns(struct vfsmount *mnt);
 extern struct mnt_namespace *copy_mnt_ns(unsigned long, struct mnt_namespace *,
 		struct fs_struct *);
 extern void put_mnt_ns(struct mnt_namespace *ns);