selftests: openvswitch: Support implicit ipv6 arguments.

The current iteration of IPv6 support requires explicit fields to be set
in addition to not properly support the actual IPv6 addresses properly.
With this change, make it so that the ipv6() bare option is usable to
create wildcarded flows to match broad swaths of ipv6 traffic.

Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Simon Horman <horms@kernel.org>
Signed-off-by: Aaron Conole <aconole@redhat.com>
Link: https://patch.msgid.link/20240625172245.233874-6-aconole@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>