bcache: revert replacing IS_ERR_OR_NULL with IS_ERR again

In the Linux kernel, the following vulnerability has been resolved: bcache: revert replacing IS_ERR_OR_NULL with IS_ERR again Commit 028ddcac477b ("bcache: Remove unnecessary NULL point check in node allocations") leads a NULL pointer deference in cache_set_flush(). 1721 if (!IS_ERR_OR_NULL(c->root)) 1722 list_add(&c->root->list, &c->btree_cache); >From the above code in cache_set_flush(), if previous registration code fails before allocating c->root, it is possible c->root is NULL as what it is initialized. __bch_btree_node_alloc() never returns NULL but c->root is possible to be NULL at above line 1721. This patch replaces IS_ERR() by IS_ERR_OR_NULL() to fix this.