block/mq-deadline: Remove a WARN_ON_ONCE() call

The purpose of the WARN_ON_ONCE() statement in dd_insert_request() is to
verify that dd_prepare_request() cleared rq->elv.priv[0]. Since
dd_prepare_request() is called during request initialization but not if a
request is requeued, a warning is triggered if a request is requeued. Fix
this by removing the WARN_ON_ONCE() statement. This patch suppresses the
following kernel warning:

WARNING: CPU: 28 PID: 432 at block/mq-deadline-main.c:740 dd_insert_request+0x4d4/0x5b0
Workqueue: kblockd blk_mq_requeue_work
Call Trace:
 dd_insert_requests+0xfa/0x130
 blk_mq_sched_insert_request+0x22c/0x240
 blk_mq_requeue_work+0x21c/0x2d0
 process_one_work+0x4c2/0xa70
 worker_thread+0x2e5/0x6d0
 kthread+0x21c/0x250
 ret_from_fork+0x1f/0x30

Reported-by: Sachin Sant <sachinp@linux.vnet.ibm.com>
Fixes: 08a9ad8b

 ("block/mq-deadline: Add cgroup support")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20210627211112.12720-1-bvanassche@acm.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>