Fixed my WRITE_DMA problems by removing vinum

I recently became one of the growing number of users seeing
WRITE_DMA-related panics on a regular basis any time I loaded down my
system.  I'd tried the workaround of running my main 120GB drive in PIO4
mode, but it was unbearably slow and the system crawled during large
transfers.

I'd been installing FreeBSD on vinum concat volumes for several years for
the flexibility in allocating storage (want to grow /usr?  Just tack another
plex to the end and growfs it).  However, I'd noticed that my storage
requirements had pretty well stabilized over the last couple of years, and
I decided to remove my vinum setup in favor of traditional partitions.

When I did that, my WRITE_DMA problems disappeared completely under all
circumstances that I've been able to test.  I have been totally unable to
panic my system (or get any kind of DMA warnings) since the migration.

I'm not ready to completely blame vinum for destabilizing my ATA system - I
wouldn't testify in court that I didn't inadvertently change any seemingly
minor but actually important setting - but the circumstantial evidence seems
pretty strong for me.
-- 
Kirk Strauser