bpf, doc: update bpf maintainers entry

Add various related files that have been missing under
BPF entry covering essential parts of its infrastructure
and also add myself as co-maintainer.

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