bpf_progs/offload.c - reduce memory usage by shrinking maps

Now that stats are cleared as soon as we turn off tethering
we no longer need to have 1000 entries in the per-interface
stats and limit maps.  In practice we'll usually have zero
(tethering inactive) or one (the active upstream tethering interface).

Test: atest
Bug: 150736748
Change-Id: I22447efac365baef8aa212a5c4732fed5c4ff98e