ovl: don't poison cursor

ovl_cache_put() can be called from ovl_dir_reset() if the cache needs to be
rebuilt.  We did list_del() on the cursor, which results in an Oops on the
poisoned pointer in ovl_seek_cursor().

Reported-by: Jordi Pujol Palomer <jordipujolp@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Tested-by: Jordi Pujol Palomer <jordipujolp@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
index 4e9d7c1..2a7ef4f 100644
--- a/fs/overlayfs/readdir.c
+++ b/fs/overlayfs/readdir.c
@@ -168,7 +168,7 @@
 {
 	struct ovl_dir_cache *cache = od->cache;
 
-	list_del(&od->cursor.l_node);
+	list_del_init(&od->cursor.l_node);
 	WARN_ON(cache->refcount <= 0);
 	cache->refcount--;
 	if (!cache->refcount) {