vfs: umount_tree() might be called on subtree that had never made it

__mnt_make_shortterm() in there undoes the effect of __mnt_make_longterm()
we'd done back when we set ->mnt_ns non-NULL; it should not be done to
vfsmounts that had never gone through commit_tree() and friends.  Kudos to
lczerner for catching that one...

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namespace.c

@@ -1074,8 +1074,9 @@ void umount_tree(struct mount *mnt, int propagate, struct list_head *kill)
 		list_del_init(&p->mnt_expire);
 		list_del_init(&p->mnt_list);
 		__touch_mnt_namespace(p->mnt_ns);
-		p->mnt_ns = NULL;
+		if (p->mnt_ns)
 			__mnt_make_shortterm(p);
+		p->mnt_ns = NULL;
 		list_del_init(&p->mnt_child);
 		if (mnt_has_parent(p)) {
 			p->mnt_parent->mnt_ghosts++;