stop abusing netd's DAC override on prog accesses by using R/O fetch

and also use mapRetrieveRW(x) instead of bpfFdGet(x, 0) or mapRetrieve(x, 0)

Test: builds, treehugger, see above
Bug: 150040815
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Original-Change: https://android-review.googlesource.com/1339962
Merged-In: I09206bd07eb3ecea5256422ed9b52b791079f75a
Change-Id: I09206bd07eb3ecea5256422ed9b52b791079f75a