brd: decrease the number of allocated pages which discarded

The number of allocated pages which discarded will not decrease.
Fix it.

Fixes: 9ead7ef ("brd: implement discard support")

Signed-off-by: Zhang Xianwei <zhang.xianwei8@zte.com.cn>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20241128170056565nPKSz2vsP8K8X2uk2iaDG@zte.com.cn
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: zhangxianwei8

drivers/block/brd.c

@@ -231,8 +231,10 @@ static void brd_do_discard(struct brd_device *brd, sector_t sector, u32 size)
 	xa_lock(&brd->brd_pages);
 	while (size >= PAGE_SIZE && aligned_sector < rd_size * 2) {
 		page = __xa_erase(&brd->brd_pages, aligned_sector >> PAGE_SECTORS_SHIFT);
-		if (page)
+		if (page) {
 			__free_page(page);
+			brd->brd_nr_pages--;
+		}
 		aligned_sector += PAGE_SECTORS;
 		size -= PAGE_SIZE;
 	}