xen-netback: using kfree_rcu() to simplify the code

#include <linux/vmalloc.h>

#include <linux/rculist.h>

static void xenvif_del_hash(struct rcu_head *rcu)

{

	struct xenvif_hash_cache_entry *entry;

	entry = container_of(rcu, struct xenvif_hash_cache_entry, rcu);

	kfree(entry);

}

static void xenvif_add_hash(struct xenvif *vif, const u8 *tag,

			    unsigned int len, u32 val)

{

		if (++vif->hash.cache.count > xenvif_hash_cache_size) {

			list_del_rcu(&oldest->link);

			vif->hash.cache.count--;

			call_rcu(&oldest->rcu, xenvif_del_hash);

			kfree_rcu(oldest, rcu);

		}

	}

	list_for_each_entry_rcu(entry, &vif->hash.cache.list, link) {

		list_del_rcu(&entry->link);

		vif->hash.cache.count--;

		call_rcu(&entry->rcu, xenvif_del_hash);

		kfree_rcu(entry, rcu);

	}

	spin_unlock_irqrestore(&vif->hash.cache.lock, flags);