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>
block: reorder elevator switch sequence

Elevator switch sequence first attached the new elevator, then tried
registering it (sysfs) and if that failed attached back the old
elevator.  However, sysfs registration doesn't require the elevator to
be attached, so there is no reason to do the "detach, attach new,
register, maybe re-attach old" sequence.  It can just do "register,
detach, attach".

* elevator_init_queue() is updated to set ->elevator_data directly and
  return 0 / -errno.  This allows elevator_exit() on an unattached
  elevator.

* __elv_unregister_queue() which was necessary to unregister
  unattached q is removed in favor of __elv_register_queue() which can
  register unattached q.

* elevator_attach() becomes a single assignment and obscures more then
  it helps.  Dropped.

This will help cleaning up io_context handling across elevator switch.

This patch doesn't introduce visible behavior change.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
fix missing code in makefile

Conflicts:
	arch/arm/boot/compressed/Makefile