futex: Mark expected switch fall-throughs

In preparation of enabling -Wimplicit-fallthrough, mark switch cases which
fall through.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Darren Hart <dvhart@infradead.org>
Link: https://lkml.kernel.org/r/20180816172124.GA2407@embeddedor.com

kernel/futex.c

@@ -3523,10 +3523,12 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
 	switch (cmd) {
 	case FUTEX_WAIT:
 		val3 = FUTEX_BITSET_MATCH_ANY;
+		/* fall through */
 	case FUTEX_WAIT_BITSET:
 		return futex_wait(uaddr, flags, val, timeout, val3);
 	case FUTEX_WAKE:
 		val3 = FUTEX_BITSET_MATCH_ANY;
+		/* fall through */
 	case FUTEX_WAKE_BITSET:
 		return futex_wake(uaddr, flags, val, val3);
 	case FUTEX_REQUEUE: