Panic in prison_alloc() on boot

[Ryan Stone]

To close the loop on this, the root cause ended up being a mistake on
my end.  This system had a rather convoluted boot process, and as a
result of that was loading a nullfs.ko built for a months-old kernel.
This setup accidentally worked for some time, but I guess some recent
change to struct thread changed the ABI, causing the old nullfs.ko to
be incompatible and fail to boot.

Sorry for the noise,
Ryan