bcache: revert replacing IS_ERR_OR_NULL with IS_ERR again

Commit 028ddca ("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.

Fixes: 028ddca ("bcache: Remove unnecessary NULL point check in node allocations")
Signed-off-by: Liequan Che <[email protected]>
Cc: [email protected]
Cc: Zheng Wang <[email protected]>
Reviewed-by: Mingzhe Zou <[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: Liequan Che

drivers/md/bcache/super.c

@@ -1718,7 +1718,7 @@ static CLOSURE_CALLBACK(cache_set_flush)
 	if (!IS_ERR_OR_NULL(c->gc_thread))
 		kthread_stop(c->gc_thread);
 
-	if (!IS_ERR(c->root))
+	if (!IS_ERR_OR_NULL(c->root))
 		list_add(&c->root->list, &c->btree_cache);
 
 	/*