block: Merge bio before checking ->cached_rq

block: Merge bio before checking ->cached_rq

It checks if plug->cached_rq is empty before merging bio. But the merge action
doesn't have relationship with plug->cached_rq, it trys to merge bio with
requests within plug->mq_list. Now it checks if ->cached_rq is empty before
merging bio. If it's empty, it will miss the merge chances. So move the merge
function before checking ->cached_rq.

Signed-off-by: Xiao Ni <xni@redhat.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230209031930.27354-1-xni@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>