blk-mq: use BLK_POLL_ONESHOT for synchronous poll completion

blk_execute_rq() with polling is used in kernel code paths such as
NVMe controller connect. The aggressive spinning in blk_hctx_poll()
can prevent the completion task from getting a chance to run, causing
a lockup.

The spinning with cpu_relax() doesn't yield CPU, so need_resched()
only becomes true on timer tick. This causes unnecessary spinning
while the completion task is already waiting to run.

Before commit f22ecf9c14c1, the loop would exit early because
task_is_running() was always true. After that commit removed the
check, the loop now spins until need_resched().

Fix this by using BLK_POLL_ONESHOT in blk_rq_poll_completion(). This
causes blk_hctx_poll() to poll once and return immediately, letting
the outer loop's cond_resched() yield CPU so the completion task can
run.

Fixes: f22ecf9c14c1 ("blk-mq: delete task running check in blk_hctx_poll()")
Cc: Diangang Li <[e-mail]>
Cc: Fengnan Chang <[e-mail]>
Reported-by: Yi Zhang <[e-mail]>
Signed-off-by: Ming Lei <[e-mail]>
Tested-by: Yi Zhang <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

block/blk-mq.c

···
 static void blk_rq_poll_completion(struct request *rq, struct completion *wait)
 {
 	do {
-		blk_hctx_poll(rq->q, rq->mq_hctx, NULL, 0);
+		blk_hctx_poll(rq->q, rq->mq_hctx, NULL, BLK_POLL_ONESHOT);
 		cond_resched();
 	} while (!completion_done(wait));
 }