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

The Netlink interface for metering was restricted to global CAP_NET_ADMIN
by using GENL_ADMIN_PERM. To allow metring in a non-inital user namespace,
e.g., a container, this is changed to GENL_UNS_ADMIN_PERM.

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