bpf: move IP_{DF,MF,...} and 'struct frag_hdr' into bpf_net_helpers.h am: 5161c30608 am: d218612cb0

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Connectivity/+/3226111

Change-Id: I1c6fee14e3d918a4d9f64bbc5b3ffe496c79657f
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>