gmirror panic on RELENG_8

[Steve Price]

I installed 8.2-RC1 on a machine with 3 identical drives and got a
kernel panic when trying to load geom_mirror on boot.  Try as I
might I've been unable to get a coredump so I can investigate further.

Minimum required to reproduce the error here is to install 8.2-RC1
from CD and on reboot, drop to the boot loader prompt and type:

load geom_mirror
boot

da0 contains all of the installed files, da1 was to be a mirror of da0
so was left alone, and da2 was created as one big partition.

I first noticed this when I installed 8.1-RC1 and then ran the following
commands after rebooting from the initial install:

sysctl kern.geom.debugflags=17
gmirror label -vb round-robin gm0 /dev/da0
echo 'geom_mirror_load="YES"' >> /boot/loader.conf
sed -i.bak -e's,da0,mirror/gm0,g' /etc/fstab
reboot

When the box came back up I got a panic.  I've tried various combos
to figure out what the minimum was to reproduce this.  I've verified
that 8.0-RELEASE, and 8.1-RELEASE don't exhibit this same behavior.

If further info is required, please let me know.

-steve