fix old umount_tree() breakage

Expiry-related code calls umount_tree() several times with
the same list to collect vfsmounts to.  Which is fine, except
that umount_tree() implicitly assumed that the list would
be empty on each call - it moves the victims over there and
then iterates through the list kicking them out.  It's *almost*
idempotent, so everything nearly worked.  However, mnt->ghosts
handling (and thus expirability checks) had been broken - that
part was not idempotent...

The fix is trivial - use local temporary list, splice it to
the the collector list when we are through.
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

fs/namespace.c

@@ -1226,15 +1226,16 @@ void release_mounts(struct list_head *head)
  */
 void umount_tree(struct vfsmount *mnt, int propagate, struct list_head *kill)
 {
+	LIST_HEAD(tmp_list);
 	struct vfsmount *p;
 
 	for (p = mnt; p; p = next_mnt(p, mnt))
-		list_move(&p->mnt_hash, kill);
+		list_move(&p->mnt_hash, &tmp_list);
 
 	if (propagate)
-		propagate_umount(kill);
+		propagate_umount(&tmp_list);
 
-	list_for_each_entry(p, kill, mnt_hash) {
+	list_for_each_entry(p, &tmp_list, mnt_hash) {
 		list_del_init(&p->mnt_expire);
 		list_del_init(&p->mnt_list);
 		__touch_mnt_namespace(p->mnt_ns);
@@ -1246,6 +1247,7 @@ void umount_tree(struct vfsmount *mnt, int propagate, struct list_head *kill)
 		}
 		change_mnt_propagation(p, MS_PRIVATE);
 	}
+	list_splice(&tmp_list, kill);
 }
 
 static void shrink_submounts(struct vfsmount *mnt, struct list_head *umounts);