blk-mq: move blk_mq_attempt_bio_merge out blk_mq_get_new_requests

blk_mq_attempt_bio_merge has nothing to do with allocating a new
request, it avoids allocating a new request.  Move the call out of
blk_mq_get_new_requests and into the only caller.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Tested-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20240124092658.2258309-2-hch@lst.de


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