dm thin: do not set discard_zeroes_data

The dm thin pool target claims to support the zeroing of discarded
data areas.  This turns out to be incorrect when processing discards
that do not exactly cover a complete number of blocks, so the target
must always set discard_zeroes_data_unsupported.

The thin pool target will zero blocks when they are allocated if the
skip_block_zeroing feature is not specified.  The block layer
may send a discard that only partly covers a block.  If a thin pool
block is partially discarded then there is no guarantee that the
discarded data will get zeroed before it is accessed again.
Due to this, thin devices cannot claim discards will always zero data.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Joe Thornber <ejt@redhat.com>
Cc: stable@vger.kernel.org # 3.4+
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm-thin.c

@@ -2274,6 +2274,7 @@ static int pool_ctr(struct dm_target *ti, unsigned argc, char **argv)
 		 * thin devices' discard limits consistent).
 		 */
 		ti->discards_supported = true;
+		ti->discard_zeroes_data_unsupported = true;
 	}
 	ti->private = pt;
 
@@ -2745,7 +2746,6 @@ static void set_discard_limits(struct pool *pool, struct queue_limits *limits)
 	 * boundary is not sent to this target.
 	 */
 	limits->discard_granularity = pool->sectors_per_block << SECTOR_SHIFT;
-	limits->discard_zeroes_data = pool->pf.zero_new_blocks;
 }
 
 static void pool_io_hints(struct dm_target *ti, struct queue_limits *limits)