Derive mtu from ipv6 route to ipv4 /96 prefix instead of device

Test: builds, atest, still needs testing on real network
  x86 clatd_test
  --------------
  clatd_test (19 Tests)
  ...
  [6/19] ClatdTest#DetectMtu: PASSED (0ms)
  ...
Bug: 147935930
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I3d11ba082dabf70089867146acd84f6436953663
diff --git a/main.c b/main.c
index 9e796fd..e973bf3 100644
--- a/main.c
+++ b/main.c
@@ -142,7 +142,7 @@
   // following line causes XLAT failure in permissive mode.
   unsetenv("ANDROID_DNS_MODE");
 
-  configure_interface(uplink_interface, plat_prefix, v4_addr, v6_addr, &tunnel, net_id);
+  configure_interface(uplink_interface, plat_prefix, v4_addr, v6_addr, &tunnel, net_id, mark);
 
   // Drop all remaining capabilities.
   set_capability(0);