Revert "bpf: Move cgroup iterator helpers to bpf.h"

This reverts commit f1f241ee which is
commit 9621e60f upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: Id3036323fb743e3a437aa0137ef311616d45aad5
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>