selftests: openvswitch: Fix the ct_tuple for v4

The ct_tuple v4 data structure decode / encode routines were using
the v6 IP address decode and relying on default encode. This could
cause exceptions during encode / decode depending on how a ct4
tuple would appear in a netlink message.

Caught during code review.

Fixes: e52b07aa

 ("selftests: openvswitch: add flow dump support")
Signed-off-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>