Increase the lowest possible good PMTU

The current minimum PMTU we can detect is eight bytes, which, being
lower than the minimum IPv4 MTU of 68 bytes, can cause an exception in
the sleeve packet fragmentation implementation. We increase that minimum
to 552, the lowest PMTU value that Linux will cache.

router/sleeve.go

@@ -917,7 +917,7 @@ func (fwd *sleeveForwarder) processSendError(err error) error {
 			return nil
 		}
 
-		fwd.mtuHighestGood = 8
+		fwd.mtuHighestGood = 552
 		fwd.mtuLowestBad = mtu + 1
 		fwd.mtuCandidate = mtu
 		fwd.mtuTestsSent = 0