dm-buffered: fix discard

Use dm_bufio_forget_buffers instead of dm_bufio_forget.
Fix calculation of "end".

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

---
 drivers/md/dm-buffered-target.c |   11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

Index: linux-2.6/drivers/md/dm-buffered-target.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-buffered-target.c
+++ linux-2.6/drivers/md/dm-buffered-target.c
@@ -158,14 +158,13 @@ static void _io(struct buffered_c *bc, s
  */
 static void _discard_blocks(struct buffered_c *bc, struct bio *bio)
 {
-	sector_t block, end, start;
+	sector_t start, end;
 
 	start = _to_block(bc, bio->bi_iter.bi_sector);
 	end = _to_block(bc, bio_end_sector(bio));
-	for (block = start ; block <= end; block++) {
-		dm_bufio_forget(bc->bufio, block);
-		cond_resched();
-	}
+	if (_sector_mod(bc, bio_end_sector(bio)))
+		end++;
+	dm_bufio_forget_buffers(bc->bufio, start, end - start);
 
 	if (bc->discard_passdown) {
 		if (_sector_mod(bc, bio->bi_iter.bi_sector))
@@ -173,7 +172,7 @@ static void _discard_blocks(struct buffe
 		if (_sector_mod(bc, bio_end_sector(bio)))
 			end--;
 		if (end >= start)
-			dm_bufio_issue_discard(bc->bufio, start,  end - start + 1);
+			dm_bufio_issue_discard(bc->bufio, start, end - start + 1);
 	}
 }