vfs_cache panic, 7.2-prerelease (stable)

[xorquewasp at googlemail.com]

Hello.

After extensive hardware testing, new thermal compound, new case and a
lot of work improving airflow, I'm now confident my new machine is OK
from a hardware point of view (a weeks worth of memory testing, days
of running prime95, temperature monitoring, extensive sessions with
sysutils/stress running from a liveCD).

About 20 minutes ago, I booted into the version of FreeBSD I had built
previously to try to debug a DRI problem (which turned out not to exist,
evidently).  The build has every debugging option I could find enabled
(WITNESS, INVARIANTS, etc). Upon trying to build the 'lmmon' port, I was
greeted with a panic. I was dropped into ddb and thinking that the
relevent crash info would be saved, I rebooted. It wasn't saved.

The panic is reproducable: it always occurs in vfs_cache.c:345.

How can I get all the info I need saved in order to file a PR?

thanks,
xw