bpf: fix bpf_skb_in_cgroup helper naming

While hashing out BPF's current_task_under_cgroup helper bits, it came
to discussion that the skb_in_cgroup helper name was suboptimally chosen.

Tejun says:

  So, I think in_cgroup should mean that the object is in that
  particular cgroup while under_cgroup in the subhierarchy of that
  cgroup. Let's rename the other subhierarchy test to under too. I
  think that'd be a lot less confusing going forward.

  [...]

  It's more intuitive and gives us the room to implement the real
  "in" test if ever necessary in the future.

Since this touches uapi bits, we need to change this as long as v4.8
is not yet officially released. Thus, change the helper enum and rename
related bits.

Fixes: 4a482f34af ("cgroup: bpf: Add bpf_skb_in_cgroup_proto")
Reference: http://patchwork.ozlabs.org/patch/658500/
Suggested-by: Sargun Dhillon <sargun@sargun.me>
Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>

include/uapi/linux/bpf.h

@@ -339,7 +339,7 @@ enum bpf_func_id {
 	BPF_FUNC_skb_change_type,
 
 	/**
-	 * bpf_skb_in_cgroup(skb, map, index) - Check cgroup2 membership of skb
+	 * bpf_skb_under_cgroup(skb, map, index) - Check cgroup2 membership of skb
 	 * @skb: pointer to skb
 	 * @map: pointer to bpf_map in BPF_MAP_TYPE_CGROUP_ARRAY type
 	 * @index: index of the cgroup in the bpf_map
@@ -348,7 +348,7 @@ enum bpf_func_id {
 	 *   == 1 skb succeeded the cgroup2 descendant test
 	 *    < 0 error
 	 */
-	BPF_FUNC_skb_in_cgroup,
+	BPF_FUNC_skb_under_cgroup,
 
 	/**
 	 * bpf_get_hash_recalc(skb)