bpf: cgroup: properly use bpf_prog_array api

Now that we don't have __rcu markers on the bpf_prog_array helpers,
let's use proper rcu_dereference_protected to obtain array pointer
under mutex.

We also don't need __rcu annotations on cgroup_bpf.inactive since
it's not read/updated concurrently.

v4:
* drop cgroup_rcu_xyz wrappers and use rcu APIs directly; presumably
  should be more clear to understand which mutex/refcount protects
  each particular place

v3:
* amend cgroup_rcu_dereference to include percpu_ref_is_dying;
  cgroup_bpf is now reference counted and we don't hold cgroup_mutex
  anymore in cgroup_bpf_release

v2:
* replace xchg with rcu_swap_protected

Cc: Roman Gushchin <guro@fb.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

include/linux/bpf-cgroup.h

@@ -72,7 +72,7 @@ struct cgroup_bpf {
 	u32 flags[MAX_BPF_ATTACH_TYPE];
 
 	/* temp storage for effective prog array used by prog_attach/detach */
-	struct bpf_prog_array __rcu *inactive;
+	struct bpf_prog_array *inactive;
 
 	/* reference counter used to detach bpf programs after cgroup removal */
 	struct percpu_ref refcnt;