ebpf tethering - add a map to store data limit, part 3

This actually starts enforcing the limit.

Note: this requires netd to populate both stats & limit maps
for offload to work.

Bug: 150736748
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I13c21f455155480712fba9b1464b4392d5169a52