blk-mq: don't insert passthrough request into sw queue

blk-mq: don't insert passthrough request into sw queue

In case of real io scheduler, q->elevator is set, so blk_mq_run_hw_queue()
may just check if scheduler queue has request to dispatch, see
__blk_mq_sched_dispatch_requests(). Then IO hang may be caused because
all passthorugh requests may stay in sw queue.

And any passthrough request should have been inserted to hctx->dispatch
always.

Reported-by: Guangwu Zhang <guazhang@redhat.com>
Fixes: d97217e7f024 ("blk-mq: don't queue plugged passthrough requests into scheduler")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230621132208.1142318-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>