net/xen-netback: prevent UAF in xenvif_flush_hash()

In the Linux kernel, the following vulnerability has been resolved: net/xen-netback: prevent UAF in xenvif_flush_hash() During the list_for_each_entry_rcu iteration call of xenvif_flush_hash, kfree_rcu does not exist inside the rcu read critical section, so if kfree_rcu is called when the rcu grace period ends during the iteration, UAF occurs when accessing head->next after the entry becomes free. Therefore, to solve this, you need to change it to list_for_each_entry_safe.