net: Add support for batching network namespace cleanups

- Add exit_list to struct net to support building lists of network
  namespaces to cleanup.

- Add exit_batch to pernet_operations to allow running operations only
  once during a network namespace exit.  Instead of once per network
  namespace.

- Factor opt ops_exit_list and ops_exit_free so the logic with cleanup
  up a network namespace does not need to be duplicated.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/net_namespace.h

@@ -43,6 +43,7 @@ struct net {
 #endif
 	struct list_head	list;		/* list of network namespaces */
 	struct list_head	cleanup_list;	/* namespaces on death row */
+	struct list_head	exit_list;	/* Use only net_mutex */
 
 	struct proc_dir_entry 	*proc_net;
 	struct proc_dir_entry 	*proc_net_stat;
@@ -236,6 +237,7 @@ struct pernet_operations {
 	struct list_head list;
 	int (*init)(struct net *net);
 	void (*exit)(struct net *net);
+	void (*exit_batch)(struct list_head *net_exit_list);
 	int *id;
 	size_t size;
 };