null_blk: implement REQ_OP_ZONE_RESET_ALL

This patch implements newly introduced zone reset all operation for
null_blk driver.

Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>