treewide: Use fallthrough pseudo-keyword

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1]. Also, remove unnecessary
fall-through markings when it is the case.

[1] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

kernel/time/posix-timers.c

@@ -439,12 +439,12 @@ static struct pid *good_sigevent(sigevent_t * event)
 		rtn = pid_task(pid, PIDTYPE_PID);
 		if (!rtn || !same_thread_group(rtn, current))
 			return NULL;
-		/* FALLTHRU */
+		fallthrough;
 	case SIGEV_SIGNAL:
 	case SIGEV_THREAD:
 		if (event->sigev_signo <= 0 || event->sigev_signo > SIGRTMAX)
 			return NULL;
-		/* FALLTHRU */
+		fallthrough;
 	case SIGEV_NONE:
 		return pid;
 	default: