bpf: Memcg-based memory accounting for bpf ringbuffer

Enable the memcg-based memory accounting for the memory used by
the bpf ringbuffer.

Signed-off-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20201201215900.3569844-15-guro@fb.com