Processing an incoming ICMPv6 we need to find just the matching inpcb and
call udp_notify() on it.
The call to in6_pcbnotify() at the first glance seems like a notifying
sweep across the entire database, but it is not. It finds just the
matching inpcb, but does that in the most inefficient way! The added test
case proves that ICMPv6 processing stays the same before and after the
change. This weird use of in6_pcbnotify() can be tracked down all the way
to the initial KAME code.