null_blk: discard zones on reset

When memory backing is enabled, use null_handle_discard() to free the
backing memory used by a zone when the zone is being reset.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>