hotfix(pmtud/icmp): set IPv6 dont fragment options just in case

internal/pmtud/icmp/df_unspecified.go

@@ -0,0 +1,10 @@
+//go:build !linux && !windows
+
+package icmp
+
+// setDontFragment for platforms other than Linux and Windows
+// is not implemented, so we just return assuming the don't
+// fragment flag is set on IP packets.
+func setDontFragment(fd uintptr, ipv4 bool) (err error) {
+	return nil
+}