cfq: Annotate fall-through in a switch statement

This patch avoids that gcc complains about fall-through when building
with W=1.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 82b6c27..ec6acdd 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -3666,6 +3666,7 @@
 	switch (ioprio_class) {
 	default:
 		printk(KERN_ERR "cfq: bad prio %x\n", ioprio_class);
+		/* fall through */
 	case IOPRIO_CLASS_NONE:
 		/*
 		 * no prio set, inherit CPU scheduling settings