bpf: Refine memcg-based memory accounting for sockmap and sockhash maps

Include internal metadata into the memcg-based memory accounting.
Also include the memory allocated on updating an element.

Signed-off-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20201201215900.3569844-17-guro@fb.com