[GENETLINK]: Relax dances with genl_lock.

The genl_unregister_family() calls the genl_unregister_mc_groups(), 
which takes and releases the genl_lock and then locks and releases
this lock itself.

Relax this behavior, all the more so the genl_unregister_mc_groups() 
is called from genl_unregister_family() only.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>