selftests/bpf: Fix v4_to_v6 in sk_lookup

I'm getting some garbage in bytes 8 and 9 when doing conversion
from sockaddr_in to sockaddr_in6 (leftover from AF_INET?). Let's
explicitly clear the higher bytes.

Fixes: 0ab5539f

 ("selftests/bpf: Tests for BPF_SK_LOOKUP attach point")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/20200807223846.4190917-1-sdf@google.com