netlink: make nlmsg_end() and genlmsg_end() void

Contrary to common expectations for an "int" return, these functions
return only a positive value -- if used correctly they cannot even
return 0 because the message header will necessarily be in the skb.

This makes the very common pattern of

  if (genlmsg_end(...) < 0) { ... }

be a whole bunch of dead code. Many places also simply do

  return nlmsg_end(...);

and the caller is expected to deal with it.

This also commonly (at least for me) causes errors, because it is very
common to write

  if (my_function(...))
    /* error condition */

and if my_function() does "return nlmsg_end()" this is of course wrong.

Additionally, there's not a single place in the kernel that actually
needs the message length returned, and if anyone needs it later then
it'll be very easy to just use skb->len there.

Remove this, and make the functions void. This removes a bunch of dead
code as described above. The patch adds lines because I did

-	return nlmsg_end(...);
+	nlmsg_end(...);
+	return 0;

I could have preserved all the function's return values by returning
skb->len, but instead I've audited all the places calling the affected
functions and found that none cared. A few places actually compared
the return value with <= 0 in dump functionality, but that could just
be changed to < 0 with no change in behaviour, so I opted for the more
efficient version.

One instance of the error I've made numerous times now is also present
in net/phonet/pn_netlink.c in the route_dumpit() function - it didn't
check for <0 or <=0 and thus broke out of the loop every single time.
I've preserved this since it will (I think) have caused the messages to
userspace to be formatted differently with just a single message for
every SKB returned to userspace. It's possible that this isn't needed
for the tools that actually use this, but I don't even know what they
are so couldn't test that changing this behaviour would be acceptable.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc drivers/acpi/event.c
Simple merge

diff --cc drivers/net/ethernet/rocker/rocker.c
Simple merge

diff --cc drivers/net/vxlan.c
Simple merge

diff --cc drivers/net/wireless/mac80211_hwsim.c
Simple merge

diff --cc drivers/scsi/pmcraid.c
Simple merge

diff --cc drivers/target/target_core_user.c
Simple merge

diff --cc drivers/thermal/thermal_core.c
Simple merge

diff --cc fs/dlm/netlink.c
Simple merge

diff --cc include/net/genetlink.h
Simple merge

diff --cc include/net/netlink.h
Simple merge

diff --cc kernel/taskstats.c
Simple merge

diff --cc net/bridge/br_fdb.c
Simple merge

diff --cc net/bridge/br_mdb.c
Simple merge

diff --cc net/bridge/br_netlink.c
Simple merge

diff --cc net/can/gw.c
Simple merge

diff --cc net/core/fib_rules.c
Simple merge

diff --cc net/core/neighbour.c
Simple merge

diff --cc net/core/rtnetlink.c
Simple merge

diff --cc net/decnet/dn_dev.c
Simple merge

diff --cc net/decnet/dn_route.c
Simple merge

diff --cc net/decnet/dn_table.c
Simple merge

diff --cc net/ieee802154/netlink.c
Simple merge

diff --cc net/ieee802154/nl-mac.c
Simple merge

diff --cc net/ieee802154/nl-phy.c
Simple merge

diff --cc net/ieee802154/nl802154.c
Simple merge

diff --cc net/ipv4/devinet.c
Simple merge

diff --cc net/ipv4/fib_semantics.c
Simple merge

diff --cc net/ipv4/inet_diag.c
Simple merge

diff --cc net/ipv4/ipmr.c
Simple merge

diff --cc net/ipv4/route.c
Simple merge

diff --cc net/ipv4/tcp_metrics.c
Simple merge

diff --cc net/ipv6/addrconf.c
Simple merge

diff --cc net/ipv6/addrlabel.c
Simple merge

diff --cc net/ipv6/ip6_fib.c
Simple merge

diff --cc net/ipv6/ip6mr.c
Simple merge

diff --cc net/ipv6/route.c
Simple merge

diff --cc net/l2tp/l2tp_netlink.c
Simple merge

diff --cc net/netfilter/ipvs/ip_vs_ctl.c
Simple merge

diff --cc net/netfilter/nf_tables_api.c
Simple merge

diff --cc net/netlabel/netlabel_cipso_v4.c
Simple merge

diff --cc net/netlabel/netlabel_mgmt.c
Simple merge

diff --cc net/netlabel/netlabel_unlabeled.c
Simple merge

diff --cc net/netlink/diag.c
Simple merge

diff --cc net/netlink/genetlink.c
Simple merge

diff --cc net/nfc/netlink.c
Simple merge

diff --cc net/openvswitch/datapath.c
Simple merge

diff --cc net/packet/diag.c
Simple merge

diff --cc net/phonet/pn_netlink.c
Simple merge

diff --cc net/unix/diag.c
Simple merge

diff --cc net/wireless/nl80211.c
Simple merge

diff --cc net/xfrm/xfrm_user.c
Simple merge