netlink: add a start callback for starting a netlink dump

The start callback allows the caller to set up a context for the
dump callbacks. Presumably, the context can then be destroyed in
the done callback.

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/genetlink.h b/include/net/genetlink.h
index 1b6b6dc..43c0e77 100644
--- a/include/net/genetlink.h
+++ b/include/net/genetlink.h
@@ -114,6 +114,7 @@
  * @flags: flags
  * @policy: attribute validation policy
  * @doit: standard command callback
+ * @start: start callback for dumps
  * @dumpit: callback for dumpers
  * @done: completion callback for dumps
  * @ops_list: operations list
@@ -122,6 +123,7 @@
 	const struct nla_policy	*policy;
 	int		       (*doit)(struct sk_buff *skb,
 				       struct genl_info *info);
+	int		       (*start)(struct netlink_callback *cb);
 	int		       (*dumpit)(struct sk_buff *skb,
 					 struct netlink_callback *cb);
 	int		       (*done)(struct netlink_callback *cb);