[PATCH] bcache: revert replacing IS_ERR_OR_NULL with IS_ERR again

[Coly Li <colyli@xxxxxxx>]

From: Liequan Che <cheliequan@xxxxxxxxxx>

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);