selftests/bpf: Simplify inet_socketpair() and vsock_socketpair_connectible()

Replace implementation with a call to a generic function.

Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Tested-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Link: https://lore.kernel.org/r/20240731-selftest-sockmap-fixes-v2-3-08a0c73abed2@rbox.co


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