blk-mq: only try to run plug merge if request has same queue with incoming bio

It is obvious that io merge can't be done between two different queues, so
just try to run io merge in case of same queue.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20211102133502.3619184-2-ming.lei@redhat.com


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