bpf: move various OFFSET helpers into bpf_net_helpers.h am: ab95754e19 am: 80cbd2f016

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Connectivity/+/3226112

Change-Id: I6c9e84eb1258360f77ee327f9a04d4036a9b06e5
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>