block: Remove elevator_lock usage from blkg_conf frozen operations

Remove the acquisition and release of q->elevator_lock in the
blkg_conf_open_bdev_frozen() and blkg_conf_exit_frozen() functions. The
elevator lock is no longer needed in these code paths since commit
78c27134 ("block: move wbt_enable_default() out of queue freezing
from sched ->exit()") which introduces `disk->rqos_state_mutex` for
protecting wbt state change, and not necessary to abuse elevator_lock
for this purpose.

This change helps to solve the lockdep warning reported from Yu Kuai[1].

Pass blktests/throtl with lockdep enabled.

Links: https://lore.kernel.org/linux-block/e5e7ac3f-2063-473a-aafb-4d8d43e5576e@yukuai.org.cn/

 [1]
Fixes: commit 78c27134 ("block: move wbt_enable_default() out of queue freezing from sched ->exit()")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>