tools/bpf: sync uapi bpf.h for bpf_get_current_cgroup_id() helper

Sync kernel uapi/linux/bpf.h with tools uapi/linux/bpf.h.
Also add the necessary helper define in bpf_helpers.h.

Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>