[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/core/rtnetlink.c b/net/core/rtnetlink.c
index 9bed756..193fd86 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -462,11 +462,6 @@
 	netlink_broadcast(rtnl, skb, 0, RTNLGRP_LINK, GFP_KERNEL);
 }
 
-static int rtnetlink_done(struct netlink_callback *cb)
-{
-	return 0;
-}
-
 /* Protected by RTNL sempahore.  */
 static struct rtattr **rta_buf;
 static int rtattr_max;
@@ -533,8 +528,7 @@
 			goto err_inval;
 
 		if ((*errp = netlink_dump_start(rtnl, skb, nlh,
-						link->dumpit,
-						rtnetlink_done)) != 0) {
+						link->dumpit, NULL)) != 0) {
 			return -1;
 		}
 		rlen = NLMSG_ALIGN(nlh->nlmsg_len);