panic with mount_union in installer

[Piotr Meyer <aniou%smutek.pl@localhost>]

Hi,
I got a panic during a kind of exotic install try of -current (8.99.34)
on amd64. Due to large number of GPT partitions I quickly got a total of
19 wedges (up to dk18) with only 16 of them available in /dev (install/53992).

Manual mknod in /dev/ wasn't possible due to r/o filesystem (cd-image), 
so I made a directory in tmpfs-mounted /tmp, some device nodes inside 
and then I mounted that dir with mount_union over /dev:

mkdir /tmp/dev
cd /tmp/dev
mknod dk16 b 168 16
mknod rdk16 c 168 16
...
mount_union /tmp/dev /dev

1. newfs -O 2 name=... was successful 
2. mount name=... /targetroot/... was successful
3. and during untar process I got following panic:
   http://smutek.pl/netbsd/netbsd-8-99-34-adm64-union-crash.jpg
   http://smutek.pl/netbsd/netbsd-8-99-34-adm64-union-crash-bt.jpg

Regards,
-- 
Piotr 'aniou' Meyer