ipc: 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:

  ipc/sem.c:1683:6: warning: this statement may fall through [-Wimplicit-fallthrough=]

Link: http://lkml.kernel.org/r/20190114203608.18218-1-malat@debian.org
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

ipc/sem.c

@@ -1680,6 +1680,7 @@ static long ksys_semctl(int semid, int semnum, int cmd, unsigned long arg, int v
 	case IPC_SET:
 		if (copy_semid_from_user(&semid64, p, version))
 			return -EFAULT;
+		/* fall through */
 	case IPC_RMID:
 		return semctl_down(ns, semid, cmd, &semid64);
 	default: