dm integrity: don't re-write metadata if discarding same blocks

If we discard already discarded blocks we do not need to write discard
pattern to the metadata, because it is already there.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-integrity.c

@@ -1429,8 +1429,10 @@ static int dm_integrity_rw_tag(struct dm_integrity_c *ic, unsigned char *tag, se
 		if (op == TAG_READ) {
 			memcpy(tag, dp, to_copy);
 		} else if (op == TAG_WRITE) {
-			memcpy(dp, tag, to_copy);
-			dm_bufio_mark_partial_buffer_dirty(b, *metadata_offset, *metadata_offset + to_copy);
+			if (memcmp(dp, tag, to_copy)) {
+				memcpy(dp, tag, to_copy);
+				dm_bufio_mark_partial_buffer_dirty(b, *metadata_offset, *metadata_offset + to_copy);
+			}
 		} else {
 			/* e.g.: op == TAG_CMP */