datapath: make generic netlink group const

datapath: make generic netlink group const

Upstream commit:
    commit 48e48a70c08a8a68f8697f8b30cb83775bda8001
    Author: stephen hemminger <stephen@networkplumber.org>
    Date:   Wed Jul 16 11:25:52 2014 -0700

    openvswitch: make generic netlink group const

    Generic netlink tables can be const.

    Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
    Signed-off-by: David S. Miller <davem@davemloft.net>
The equivalent tables in meter.c and conntrack.c are constified so
it should be safe to do the same for these and will improve
security as well.

Original patch slightly modified for out of tree module.

Passes check-kmod.
Passes Travis.
https://travis-ci.org/gvrose8192/ovs-experimental/builds/616880002

Cc: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>