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/decnet/dn_nsp_in.c b/net/decnet/dn_nsp_in.c
index c68503a..c97bdca 100644
--- a/net/decnet/dn_nsp_in.c
+++ b/net/decnet/dn_nsp_in.c
@@ -483,7 +483,7 @@ static void dn_nsp_disc_conf(struct sock *sk, struct sk_buff *skb)
 		break;
 	case DN_RUN:
 		sk->sk_shutdown |= SHUTDOWN_MASK;
-		/* fall through */
+		fallthrough;
 	case DN_CC:
 		scp->state = DN_CN;
 	}