[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/ipv4/inet_diag.c b/net/ipv4/inet_diag.c
index 71f3c73..39061ed 100644
--- a/net/ipv4/inet_diag.c
+++ b/net/ipv4/inet_diag.c
@@ -724,12 +724,6 @@
 	return skb->len;
 }
 
-static int inet_diag_dump_done(struct netlink_callback *cb)
-{
-	return 0;
-}
-
-
 static __inline__ int
 inet_diag_rcv_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
 {
@@ -760,8 +754,7 @@
 				goto err_inval;
 		}
 		return netlink_dump_start(idiagnl, skb, nlh,
-					  inet_diag_dump,
-					  inet_diag_dump_done);
+					  inet_diag_dump, NULL);
 	} else {
 		return inet_diag_get_exact(skb, nlh);
 	}