blk-mq: fix an IS_ERR() vs NULL bug

The __blk_mq_alloc_disk() function doesn't return NULLs it returns
error pointers.

Fixes: b461dfc4 ("blk-mq: add the blk_mq_alloc_disk APIs")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/YMyjci35WBqrtqG+@mwandaSigned-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/blk-mq.h

@@ -431,7 +431,7 @@ enum {
 	static struct lock_class_key __key;				\
 	struct gendisk *__disk = __blk_mq_alloc_disk(set, queuedata);	\
 									\
-	if (__disk)							\
+	if (!IS_ERR(__disk))						\
 		lockdep_init_map(&__disk->lockdep_map,			\
 			"(bio completion)", &__key, 0);			\
 	__disk;								\