RDS: use kfree_rcu in rds_ib_remove_ipaddr

synchronize_rcu() slowing down un-necessarily the socket shutdown
path. It is used just kfree() the ip addresses in rds_ib_remove_ipaddr()
which is perfect usecase for kfree_rcu();

So lets use that to gain some speedup.
Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>

net/rds/ib.h

@@ -164,6 +164,7 @@ struct rds_ib_connection {
 struct rds_ib_ipaddr {
 	struct list_head	list;
 	__be32			ipaddr;
+	struct rcu_head		rcu;
 };
 
 struct rds_ib_device {

net/rds/ib_rdma.c

@@ -159,10 +159,8 @@ static void rds_ib_remove_ipaddr(struct rds_ib_device *rds_ibdev, __be32 ipaddr)
 	}
 	spin_unlock_irq(&rds_ibdev->spinlock);
 
-	if (to_free) {
-		synchronize_rcu();
-		kfree(to_free);
-	}
+	if (to_free)
+		kfree_rcu(to_free, rcu);
 }
 
 int rds_ib_update_ipaddr(struct rds_ib_device *rds_ibdev, __be32 ipaddr)