dm delay: support discard

Enable discard support for the delay target.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm-delay.c

@@ -198,6 +198,7 @@ out:
 	atomic_set(&dc->may_delay, 1);
 
 	ti->num_flush_requests = 1;
+	ti->num_discard_requests = 1;
 	ti->private = dc;
 	return 0;