net,rcu: convert call_rcu(ipv6_mc_socklist_reclaim) to kfree_rcu()

The rcu callback ipv6_mc_socklist_reclaim() just calls a kfree(),
so we use kfree_rcu() instead of the call_rcu(ipv6_mc_socklist_reclaim).

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>