random panics

[Patrick Welche <prlw1%cam.ac.uk@localhost>]

I have yet another panic, this time PG_ZERO page isn't
zero-filled.

uvm_pagezerocheck
uvm_pagefree
pmap_update
pmap_destroy
uvmspace_free

But it seems that these panics are random - next build will no
doubt work. I have been seeing these for the last few months on
-current. At one point, I think it was Paul?, mentioned that
there was a problem with nfe and 4Gb of memory. I can't see a PR
for it, and wonder whether anyone else has trouble with that
combination..

Cheers,

Patrick
(hello 5.99.01!)