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/recvmsg.c

@@ -776,7 +776,7 @@ out:
 	case RXRPC_ACK_DELAY:
 		if (ret != -EAGAIN)
 			break;
-		/* Fall through */
+		fallthrough;
 	default:
 		rxrpc_send_ack_packet(call, false, NULL);
 	}