selftests: ncdevmem: Switch to AF_INET6

Use dualstack socket to support both v4 and v6. v4-mapped-v6 address
can be used to do v4.

Reviewed-by: Mina Almasry <almasrymina@google.com>
Reviewed-by: Joe Damato <jdamato@fastly.com>
Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20241107181211.3934153-7-sdf@fomichev.me


Signed-off-by: Jakub Kicinski <kuba@kernel.org>