TetherIngressValue - add output path/route mtu field

This doesn't actually change anything, since we initialize it with
1500 default just like we blindly assume it always is today.

Tested: builds
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Ie448a988c3a769be07f895621711fe4e59d4535c