clat: switch to relative cbpf filter

Needed to support AF_PACKET/SOCK_RAW sockets

Test: TreeHugger, ping on ipv6-only wifi works
Bug: 265591307
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: If324ee1e97a90b1a0a2ee0fabe1d91ef0603fdac