bpf: cgroup: annotate pointers in struct cgroup_bpf with __rcu

The member 'effective' in 'struct cgroup_bpf' is protected by RCU.
Annotate it accordingly to squelch a sparse warning.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>