brd: decrease the number of allocated pages which discarded

brd: decrease the number of allocated pages which discarded

[ Upstream commit 82734209bedd65a8b508844bab652b464379bfdd ]

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

Fixes: 9ead7efc6f3f ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>