dm: zero silently drop discards

Have the zero target silently drop a discard rather than fail the
request with -EOPNOTSUPP.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
diff --git a/drivers/md/dm-zero.c b/drivers/md/dm-zero.c
index bbc9703..cc2b3cb 100644
--- a/drivers/md/dm-zero.c
+++ b/drivers/md/dm-zero.c
@@ -22,6 +22,11 @@
 		return -EINVAL;
 	}
 
+	/*
+	 * Silently drop discards, avoiding -EOPNOTSUPP.
+	 */
+	ti->num_discard_requests = 1;
+
 	return 0;
 }