switch mnt_hash to hlist

fixes RCU bug - walking through hlist is safe in face of element moves,
since it's self-terminating.  Cyclic lists are not - if we end up jumping
to another hash chain, we'll loop infinitely without ever hitting the
original list head.

[fix for dumb braino folded]

Spotted by: Max Kellermann <mk@cm4all.com>
Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/pnode.h b/fs/pnode.h
index 59e7eda..fc28a27 100644
--- a/fs/pnode.h
+++ b/fs/pnode.h
@@ -36,8 +36,8 @@
 
 void change_mnt_propagation(struct mount *, int);
 int propagate_mnt(struct mount *, struct mountpoint *, struct mount *,
-		struct list_head *);
-int propagate_umount(struct list_head *);
+		struct hlist_head *);
+int propagate_umount(struct hlist_head *);
 int propagate_mount_busy(struct mount *, int);
 void mnt_release_group_id(struct mount *);
 int get_dominating_id(struct mount *mnt, const struct path *root);