net: rtnl: use rcu_replace_pointer_rtnl in rtnl_unregister_*

With the introduction of the rcu_replace_pointer_rtnl helper,
cleanup the rtnl_unregister_* functions to use the helper instead
of open coding it.

Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>