block: don't protect submit_bio_checks by q_usage_counter

Commit cc9c884d ("block: call submit_bio_checks under q_usage_counter")
uses q_usage_counter to protect submit_bio_checks for avoiding IO after
disk is deleted by del_gendisk().

Turns out the protection isn't necessary, because once
blk_mq_freeze_queue_wait() in del_gendisk() returns:

1) all in-flight IO has been done

2) all new IO will be failed in __bio_queue_enter() because
   q_usage_counter is dead, and GD_DEAD is set

3) both disk and request queue instance are safe since caller of
submit_bio() guarantees that the disk can't be closed.

Once submit_bio_checks() needn't the protection of q_usage_counter, we can
move submit_bio_checks before calling blk_mq_submit_bio() and
->submit_bio(). With this change, we needn't to throttle queue with
holding one allocated request, then precise driver tag or request won't be
wasted in throttling. Meantime we can unify the bio check for both bio
based and request based driver.

Cc: Christoph Hellwig ...