selftests/bpf: Factor out udp_socketpair()

Factor out a common helper udp_socketpair() which creates
a pair of connected UDP sockets.

Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210704190252.11866-9-xiyou.wangcong@gmail.com