net: openvswitch: allow conntrack in non-initial user namespace

Similar to the previous commit, the Netlink interface of the OVS
conntrack module was restricted to global CAP_NET_ADMIN by using
GENL_ADMIN_PERM. This is changed to GENL_UNS_ADMIN_PERM to support
unprivileged containers in non-initial user namespace.

Signed-off-by: Michael Weiß <michael.weiss@aisec.fraunhofer.de>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>