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+@mwanda


Signed-off-by: Jens Axboe <axboe@kernel.dk>