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>

net/decnet/af_decnet.c

@@ -623,12 +623,12 @@ static void dn_destroy_sock(struct sock *sk)
 		goto disc_reject;
 	case DN_RUN:
 		scp->state = DN_DI;
-		/* fall through */
+		fallthrough;
 	case DN_DI:
 	case DN_DR:
 disc_reject:
 		dn_nsp_send_disc(sk, NSP_DISCINIT, 0, sk->sk_allocation);
-		/* fall through */
+		fallthrough;
 	case DN_NC:
 	case DN_NR:
 	case DN_RJ:
@@ -642,7 +642,7 @@ disc_reject:
 		break;
 	default:
 		printk(KERN_DEBUG "DECnet: dn_destroy_sock passed socket in invalid state\n");
-		/* fall through */
+		fallthrough;
 	case DN_O:
 		dn_stop_slow_timer(sk);