selftests/bpf: Use SOCKMAP for server sockets in bpf_sk_assign test

Update bpf_sk_assign test to fetch the server socket from SOCKMAP, now that
map lookup from BPF in SOCKMAP is enabled. This way the test TC BPF program
doesn't need to know what address server socket is bound to.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200429181154.479310-4-jakub@cloudflare.com