brd: Refuse improperly aligned discard requests

Currently when improperly aligned discard request is submitted, we just
silently discard more / less data which results in filesystem corruption
in some cases. Refuse such misaligned requests.

Signed-off-by: Jan Kara <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

drivers/block/brd.c

···
 		goto io_error;
 
 	if (unlikely(bio->bi_rw & REQ_DISCARD)) {
+		if (sector & ((PAGE_SIZE >> SECTOR_SHIFT) - 1) ||
+		    bio->bi_iter.bi_size & PAGE_MASK)
+			goto io_error;
 		discard_from_brd(brd, sector, bio->bi_iter.bi_size);
 		goto out;
 	}