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>
diff --git a/drivers/block/null_blk_main.c b/drivers/block/null_blk_main.c
index a223bee..b758b93 100644
--- a/drivers/block/null_blk_main.c
+++ b/drivers/block/null_blk_main.c
@@ -1076,8 +1076,8 @@ static void nullb_fill_pattern(struct nullb *nullb, struct page *page,
 	kunmap_atomic(dst);
 }
 
-static blk_status_t null_handle_discard(struct nullb_device *dev,
-					sector_t sector, sector_t nr_sectors)
+blk_status_t null_handle_discard(struct nullb_device *dev,
+				 sector_t sector, sector_t nr_sectors)
 {
 	struct nullb *nullb = dev->nullb;
 	size_t n = nr_sectors << SECTOR_SHIFT;