blk-mq: don't issue request directly in case that current is to be blocked

When flushing plug list in case that current will be blocked, we can't
issue request directly because ->queue_rq() may sleep, otherwise scheduler
may complain.

Fixes: dc5fc361

 ("block: attempt direct issue of plug list")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20211026082257.2889890-1-ming.lei@redhat.com


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