bcache: Fix __bch_btree_node_alloc to make the failure behavior consistent

In the Linux kernel, the following vulnerability has been resolved:

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.