The small blk_mq_attempt_merge() function is only called by
__blk_mq_sched_bio_merge(), just open code it.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>