Revert "block: Merge bio before checking ->cached_rq"

This reverts commit 23f3e327.

blk-mq sched bio merge still needs request to grab queue usage counter,
so we can't simply call blk_mq_attempt_bio_merge() when queue usage
counter isn't held.

Fixes: 23f3e327

 ("block: Merge bio before checking ->cached_rq")
Cc: Xiao Ni <xni@redhat.com>
Reported-by: Yi Zhang <yi.zhang@redhat.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230420112018.1108058-1-ming.lei@redhat.com


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