selftests: ncdevmem: Make client_ip optional

Support 3-tuple filtering by making client_ip optional. When -c is
not passed, don't specify src-ip/src-port in the filter.

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/20241107181211.3934153-5-sdf@fomichev.me


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