bpf: bpftool, support for sockhash

This adds the SOCKHASH map type to bpftools so that we get correct
pretty printing.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>