ANDROID: net: bpf: Allow TC programs to call BPF_FUNC_skb_change_head

This enables ebpf tethering offload from a cellular interface with
no L2 mac header to a wifi/ethernet/usb interface with one.

Will pursue upstreaming this along with further mtu related fixups.

Test: builds, real testing with identical patch on a 4.14 flame device
Bug: 149724482
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Ic182320bf1abb248d0f86fa2973c60208710f664