genetlink: fix genlmsg_multicast() bug

Unfortunately, I introduced a tremendously stupid bug into
genlmsg_multicast() when doing all those multicast group
changes: it adjusts the group number, but then passes it
to genlmsg_multicast_netns() which does that again.

Somehow, my tests failed to catch this, so add a warning
into genlmsg_multicast_netns() and remove the offending
group ID adjustment.

Also add a warning to the similar code in other functions
so people who misuse them are more loudly warned.

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