Interface routes are refcounted as packets move through the stack,
and there's garbage collection tied to it so that route changes can
safely propagate while traffic is flowing. In our setup, we weren't
changing or deleting any routes, but the refcounting logic in
ip6_input() was wrong and caused a reference leak on every inbound
V6 packet. This eventually caused a 32bit overflow, and the resulting
0 value caused the garbage collection to run on the active route.
That then snowballed into the panic.
Reviewed by: scottl
MFC after: 3 days
3df0e439b0