dm delay: support discard

Enable discard support for the delay target.

Signed-off-by: Mike Snitzer <[e-mail]>
Signed-off-by: Alasdair G Kergon <[e-mail]>

drivers/md/dm-delay.c

···
 	atomic_set(&dc->may_delay, 1);
 
 	ti->num_flush_requests = 1;
+	ti->num_discard_requests = 1;
 	ti->private = dc;
 	return 0;