selftests/bpf: Use sa_family_t everywhere in reuseport tests

Update the only function that is not using sa_family_t in this source file.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191212102259.418536-4-jakub@cloudflare.com