Set a high metric on linux routes that have a via.

See issue #750.

zerotier doesn't currently set a metric on routes. Linux
takes this to mean "0", the highest priority.

Every comment in the issue is about routing between zerotier
and lan and how they conflict.

This quick change could fix this problem for most people.

The subnet route for the zerotier network, the one with no
via, is still 0 in this patch. Just the "via" routes get
higher metrics.
If for some reason, you needed your via routes to have
higher priority, you could use a prefix work-around:

192.168.1.0/25 via 10.147.17.1
192.168.1.128/25 via 10.147.17.1

osdep/LinuxNetLink.cpp

@@ -25,6 +25,7 @@
 #ifndef IFNAMSIZ
 #define IFNAMSIZ 16
 #endif
+const int ZT_RTE_METRIC = 5000;
 
 namespace ZeroTier {
 
@@ -731,6 +732,23 @@ void LinuxNetLink::addRoute(const InetAddress &target, const InetAddress &via, c
 	rtl += rtap->rta_len;
 
 	if(via) {
+		/*
+		 *  Setting a metric keeps zerotier routes from taking priority over physical
+		 *  At best the computer would use zerotier through the router instead of the LAN.
+		 *  At worst it stops working at all.
+		 *
+		 *  default via 192.168.82.1 dev eth0 proto dhcp src 192.168.82.169 metric 202 
+		 *  10.147.17.0/24 dev zt5u4uptmb proto kernel scope link src 10.147.17.94 
+		 *  192.168.82.0/24 dev eth0 proto dhcp scope link src 192.168.82.169 metric 202 
+		 *  192.168.82.0/24 via 10.147.17.1 dev zt5u4uptmb proto static metric 5000 
+		 *  
+		*/
+		rtap = (struct rtattr*)(((char*)rtap) + rtap->rta_len);
+		rtap->rta_type = RTA_PRIORITY;
+		rtap->rta_len = RTA_LENGTH(sizeof(ZT_RTE_METRIC));
+		memcpy(RTA_DATA(rtap), &ZT_RTE_METRIC, sizeof(ZT_RTE_METRIC));
+		rtl += rtap->rta_len;
+
 		rtap = (struct rtattr *)(((char*)rtap)+rtap->rta_len);
 		rtap->rta_type = RTA_GATEWAY;
 		if(via.isV4()) {