selftests/bpf: Socket pair creation, cleanups

Following create_pair() changes, remove unused function argument in
create_socket_pairs() and adapt its callers, i.e. drop the open-coded
loopback socket creation.

Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Tested-by: Jakub Sitnicki <jakub@cloudflare.com>
Suggested-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Link: https://lore.kernel.org/r/20240731-selftest-sockmap-fixes-v2-2-08a0c73abed2@rbox.co


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>