bpf: introduce cgroup storage maps

This commit introduces BPF_MAP_TYPE_CGROUP_STORAGE maps:
a special type of maps which are implementing the cgroup storage.

>From the userspace point of view it's almost a generic
hash map with the (cgroup inode id, attachment type) pair
used as a key.

The only difference is that some operations are restricted:
  1) a user can't create new entries,
  2) a user can't remove existing entries.

The lookup from userspace is o(log(n)).

Signed-off-by: Roman Gushchin <guro@fb.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

kernel/bpf/syscall.c

@@ -957,6 +957,9 @@ static void free_used_maps(struct bpf_prog_aux *aux)
 {
 	int i;
 
+	if (aux->cgroup_storage)
+		bpf_cgroup_storage_release(aux->prog, aux->cgroup_storage);
+
 	for (i = 0; i < aux->used_map_cnt; i++)
 		bpf_map_put(aux->used_maps[i]);