bpf: Copy comment about bpffs labelling into bpf_helpers.h am: 172873f8d7

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/libs/net/+/23368635

Change-Id: I8533535554acda336a35b493ba26313069fcc7d9
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>