btrfs: annotate intentional switch case fallthroughs

This keeps static checkers happy.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 06537d2..976f6aa 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -437,6 +437,7 @@
 		case Opt_compress_force:
 		case Opt_compress_force_type:
 			compress_force = true;
+			/* Fallthrough */
 		case Opt_compress:
 		case Opt_compress_type:
 			if (token == Opt_compress ||