[NETLINK]: Make netlink_callback->done() optional

Most netlink families make no use of the done() callback, making
it optional gets rid of all unnecessary dummy implementations.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/xfrm/xfrm_user.c b/net/xfrm/xfrm_user.c
index c35336a..6996b7e 100644
--- a/net/xfrm/xfrm_user.c
+++ b/net/xfrm/xfrm_user.c
@@ -948,11 +948,6 @@
 	[XFRM_MSG_FLUSHPOLICY - XFRM_MSG_BASE] = { .doit = xfrm_flush_policy  },
 };
 
-static int xfrm_done(struct netlink_callback *cb)
-{
-	return 0;
-}
-
 static int xfrm_user_rcv_msg(struct sk_buff *skb, struct nlmsghdr *nlh, int *errp)
 {
 	struct rtattr *xfrma[XFRMA_MAX];
@@ -990,8 +985,7 @@
 			goto err_einval;
 
 		if ((*errp = netlink_dump_start(xfrm_nl, skb, nlh,
-						link->dump,
-						xfrm_done)) != 0) {
+						link->dump, NULL)) != 0) {
 			return -1;
 		}
 		rlen = NLMSG_ALIGN(nlh->nlmsg_len);