selftests/bpf: Add pairs_redir_to_connected helper

selftests/bpf: Add pairs_redir_to_connected helper

Extract duplicate code from these four functions

 unix_redir_to_connected()
 udp_redir_to_connected()
 inet_unix_redir_to_connected()
 unix_inet_redir_to_connected()

to generate a new helper pairs_redir_to_connected(). Create the
different socketpairs in these four functions, then pass the
socketpairs info to the new common helper to do the connections.

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Link: https://lore.kernel.org/r/54bb28dcf764e7d4227ab160883931d2173f4f3d.1696588133.git.geliang.tang@suse.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>