Always disable PMTU discovery

fastd currently can't fragment packets anyways, so it doesn't make any sense to
perform PMTU discovery. This makes the config option `pmtu' a no-op.

src/config.y

@@ -369,7 +369,7 @@ mtu:		TOK_UINT {
 		}
 	;
 
-pmtu:		autobool	{ conf.pmtu = $1; }
+pmtu:		autobool
 	;
 
 mode:		TOK_TAP		{ conf.mode = MODE_TAP; }

src/fastd.h

@@ -199,7 +199,6 @@ struct fastd_config {
 
 	uint32_t packet_mark;			/**< The configured packet mark (or 0) */
 	bool forward;				/**< Specifies if packet forwarding is enable */
-	fastd_tristate_t pmtu;			/**< Can be set to explicitly enable or disable PMTU detection */
 	bool secure_handshakes;			/**< Can be set to false to support connections with fastd versions before v11 */
 
 	fastd_drop_caps_t drop_caps;		/**< Specifies if and when to drop capabilities */

src/socket.c

@@ -102,12 +102,10 @@ static int bind_socket(const fastd_bind_address_t *addr, bool warn) {
 #endif
 
 #ifdef USE_PMTU
-	if (conf.pmtu.set) {
-		int pmtu = conf.pmtu.state ? IP_PMTUDISC_DO : IP_PMTUDISC_DONT;
-		if (setsockopt(fd, IPPROTO_IP, IP_MTU_DISCOVER, &pmtu, sizeof(pmtu))) {
-			pr_error_errno("setsockopt: unable to set PMTU discovery");
-			goto error;
-		}
+	int pmtu = IP_PMTUDISC_DONT;
+	if (setsockopt(fd, IPPROTO_IP, IP_MTU_DISCOVER, &pmtu, sizeof(pmtu))) {
+		pr_error_errno("setsockopt: unable to disable PMTU discovery");
+		goto error;
 	}
 #endif