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

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

Change-Id: I2e438849e3b2cd4a3a367ec2c4815e46514bc6cb
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>