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/net/atm/lec.c b/net/atm/lec.c
index 875fc0b..b570ef9 100644
--- a/net/atm/lec.c
+++ b/net/atm/lec.c
@@ -380,7 +380,7 @@ static int lec_atm_send(struct atm_vcc *vcc, struct sk_buff *skb)
 
 		if (mesg->content.normal.no_source_le_narp)
 			break;
-		/* FALL THROUGH */
+		fallthrough;
 	case l_arp_update:
 		lec_arp_update(priv, mesg->content.normal.mac_addr,
 			       mesg->content.normal.atm_addr,