ebpf tethering offload - only offload some src ips.

We don't want things like src ip ::, ::1, fe80::/64 being blindly
forwarded, since we're trying to behave like an ipv6 router.

Test: atest
Bug: 150808346
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I43da7829caa7fc831bd17ada5750dc06b276804d