loop: fix loop block driver discard and encryption comment

The loop driver does not support discard if encryption is enabled,
fix the comment.

Signed-off-by: Dave Young <dyoung@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 68b205a..5579fda 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -422,7 +422,7 @@
 
 		/*
 		 * We use punch hole to reclaim the free space used by the
-		 * image a.k.a. discard. However we do support discard if
+		 * image a.k.a. discard. However we do not support discard if
 		 * encryption is enabled, because it may give an attacker
 		 * useful information.
 		 */