disco,net/tstun,wgengine/magicsock: probe peer MTU

Automatically probe the path MTU to a peer when peer MTU is enabled, but do not
use the MTU information for anything yet.

Updates #311

Signed-off-by: Val <valerie@tailscale.com>

wgengine/magicsock/peermtu.go

@@ -5,6 +5,8 @@
 
 package magicsock
 
+import "tailscale.com/net/tstun"
+
 // Peer path MTU routines shared by platforms that implement it.
 
 // DontFragSetting returns true if at least one of the underlying sockets of
@@ -102,6 +104,9 @@ func (c *Conn) UpdatePMTUD() {
 		_ = c.setDontFragment("udp6", false)
 		newStatus = false
 	}
+	if debugPMTUD() {
+		c.logf("magicsock: peermtu: peer MTU probes are %v", tstun.WireMTUsToProbe)
+	}
 	c.peerMTUEnabled.Store(newStatus)
 	c.resetEndpointStates()
 }