block: reorganize queue draining

Reorganize queue draining related code in preparation of queue exit
changes.

* Factor out actual draining from elv_quiesce_start() to
  blk_drain_queue().

* Make elv_quiesce_start/end() responsible for their own locking.

* Replace open-coded ELVSWITCH clearing in elevator_switch() with
  elv_quiesce_end().

This patch doesn't cause any visible functional difference.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>