blkcg: annotate implicit fall through

There is a plan to build the kernel with -Wimplicit-fallthrough and
this place in the code produced a warning (W=1).

This commit remove the following warning:

  kernel/trace/blktrace.c:725:9: warning: this statement may fall through [-Wimplicit-fallthrough=]
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

kernel/trace/blktrace.c

@@ -723,6 +723,7 @@ int blk_trace_ioctl(struct block_device *bdev, unsigned cmd, char __user *arg)
 #endif
 	case BLKTRACESTART:
 		start = 1;
+		/* fall through */
 	case BLKTRACESTOP:
 		ret = __blk_trace_startstop(q, start);
 		break;