netlink: advertise incomplete dumps

Consider the following situation:
 * a dump that would show 8 entries, four in the first
   round, and four in the second
 * between the first and second rounds, 6 entries are
   removed
 * now the second round will not show any entry, and
   even if there is a sequence/generation counter the
   application will not know

To solve this problem, add a new flag NLM_F_DUMP_INTR
to the netlink header that indicates the dump wasn't
consistent, this flag can also be set on the MSG_DONE
message that terminates the dump, and as such above
situation can be detected.

To achieve this, add a sequence counter to the netlink
callback struct. Of course, netlink code still needs
to use this new functionality. The correct way to do
that is to always set cb->seq when a dumpit callback
is invoked and call nl_dump_check_consistent() for
each new message. The core code will also call this
function for the final MSG_DONE message.

To make it usable with generic netlink, a new function
genlmsg_nlhdr() is needed to obtain the netlink header
from the genetlink user header.

Signed-off-by: Johannes Berg <[email protected]>
Acked-by: David S. Miller <[email protected]>
Signed-off-by: John W. Linville <[email protected]>

net/netlink/af_netlink.c

@@ -1693,6 +1693,8 @@ static int netlink_dump(struct sock *sk)
 	if (!nlh)
 		goto errout_skb;
 
+	nl_dump_check_consistent(cb, nlh);
+
 	memcpy(nlmsg_data(nlh), &len, sizeof(len));
 
 	if (sk_filter(sk, skb))