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>
diff --git a/drivers/atm/lanai.c b/drivers/atm/lanai.c
index 986c131..ac811cf 100644
--- a/drivers/atm/lanai.c
+++ b/drivers/atm/lanai.c
@@ -2019,7 +2019,7 @@ static int lanai_normalize_ci(struct lanai_dev *lanai,
 	switch (*vpip) {
 		case ATM_VPI_ANY:
 			*vpip = 0;
-			/* FALLTHROUGH */
+			fallthrough;
 		case 0:
 			break;
 		default: