Later patch adds support for switching network namespaces before
running client, server or both. Device validations need to be
done after the network namespace switch, so add a helper to do it
and invoke in server and client code versus inline with argument
parsing. Move related argument checks as well.
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>