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

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

Change-Id: I4c21804a8511f54ff891f67e2d8d901491923675
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>