bcache: Fix __bch_btree_node_alloc to make the failure behavior consistent

In some specific situations, the return value of __bch_btree_node_alloc
may be NULL. This may lead to a potential NULL pointer dereference in
caller function like a calling chain :
btree_split->bch_btree_node_alloc->__bch_btree_node_alloc.

Fix it by initializing the return value in __bch_btree_node_alloc.

Fixes: cafe563 ("bcache: A block layer cache")
Cc: [email protected]
Signed-off-by: Zheng Wang <[email protected]>
Signed-off-by: Coly Li <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

Author: bluesheep1337

drivers/md/bcache/btree.c

@@ -1090,10 +1090,12 @@ struct btree *__bch_btree_node_alloc(struct cache_set *c, struct btree_op *op,
 				     struct btree *parent)
 {
 	BKEY_PADDED(key) k;
-	struct btree *b = ERR_PTR(-EAGAIN);
+	struct btree *b;
 
 	mutex_lock(&c->bucket_lock);
 retry:
+	/* return ERR_PTR(-EAGAIN) when it fails */
+	b = ERR_PTR(-EAGAIN);
 	if (__bch_bucket_alloc_set(c, RESERVE_BTREE, &k.key, wait))
 		goto err;