block: use {alloc|free}_sched data methods

block: use {alloc|free}_sched data methods

The previous patch introduced ->alloc_sched_data and
->free_sched_data methods. This patch builds upon that
by now using these methods during elevator switch and
nr_hw_queue update.

It's also ensured that scheduler-specific data is
allocated and freed through the new callbacks outside
of the ->freeze_lock and ->elevator_lock locking contexts,
thereby preventing any dependency on pcpu_alloc_mutex.

Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Yu Kuai <yukuai@fnnas.com>
Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>