bpf: allocate cgroup storage entries on attaching bpf programs

If a bpf program is using cgroup local storage, allocate
a bpf_cgroup_storage structure automatically on attaching the program
to a cgroup and save the pointer into the corresponding bpf_prog_list
entry.
Analogically, release the cgroup local storage on detaching
of the bpf program.

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>

include/linux/bpf-cgroup.h

@@ -43,6 +43,7 @@ struct bpf_cgroup_storage {
 struct bpf_prog_list {
 	struct list_head node;
 	struct bpf_prog *prog;
+	struct bpf_cgroup_storage *storage;
 };
 
 struct bpf_prog_array;