ebpf tethering - add a map to store data limit, part 1

This just defines the map, but doesn't populate it,
nor does it use the information yet.

Tested:
  $ adb shell ls -l /sys/fs/bpf | egrep limit
  -rw-rw---- 1 root network_stack 0 2020-05-05 02:06 map_offload_tether_limit_map

Bug: 150736748
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I665c14d8a3eeb949d5ae2f8ddb9f590040ddce10