From: Thomas Graf <tgraf@suug.ch>
Date: Thu, 27 Jul 2006 00:48:48 +0200

Fortunately it's only used read-only when notifying to figure
out the destination pid but that code seems bogus as well and
we can probably remove the netlink cb passing hell throughout
all the code there.

Yes, that pid handling looks very strange.  But I would
search the history of this code that passes the netlink
skb parms around before making any changes to things like
inet6_rt_notify :)

It appears to have been added in 2.4.32, the original patch passes the
raw pid around as the argument but that got changed to the current
form which passes the netlink_skb_parms.