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>

drivers/rtc/rtc-m41t80.c

@@ -740,7 +740,7 @@ static int wdt_ioctl(struct file *file, unsigned int cmd,
 			return -EINVAL;
 		wdt_margin = new_margin;
 		wdt_ping();
-		/* Fall through */
+		fallthrough;
 	case WDIOC_GETTIMEOUT:
 		return put_user(wdt_margin, (int __user *)arg);