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/rxrpc/local_object.c

@@ -162,7 +162,7 @@ static int rxrpc_open_socket(struct rxrpc_local *local, struct net *net)
 		/* Fall through and set IPv4 options too otherwise we don't get
 		 * errors from IPv4 packets sent through the IPv6 socket.
 		 */
-		/* Fall through */
+		fallthrough;
 	case AF_INET:
 		/* we want to receive ICMP errors */
 		ip_sock_set_recverr(local->socket->sk);