net,rcu: Convert call_rcu(xt_rateest_free_rcu) to kfree_rcu()

The RCU callback xt_rateest_free_rcu() just calls kfree(), so we can
use kfree_rcu() instead of call_rcu().  This also allows us to dispense
with an rcu_barrier() call, speeding up unloading of this module.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Patrick McHardy <kaber@trash.net>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>