more(?) failed assertions in vfs_bio.c

[Rafal Boni <rafal%pobox.com@localhost>]

Saw this trying to copy new kernels from the MSDOS partition on the CF card
to the FFS partion on the Jornada 720 (a simple cp from /mnt/msdos to /).

panic: kernel diagnostic assertion "bp->b_vp == NULL || bp->b_vp->v_tag
!= VT_UFS || ISSET(bp->b_flags, B_COWDONE)" failed: file 
"../../../../kern/vfs_bio.c", line 888

Hand-copied backtrace:
        __kernassert
        bdwrite
        msdosfs_update
        msdosfs_inactive
        VOP_INACTIVE
        vrelel
        vn_close
        closef
        fd_close
        syscall_plain

It looks to be dead-reproducible... happened in 4.99.65 and looks to be
the same failure in .67 from earlier today.  This sounds like one of the
panics Manuel was chasing, but I'm running out of time for the night so
I thought I'd lob this out here anyway.  Is this an already-reported
issue?  Should I PR it?  Dig into it more?

Machine is hpcarm, though I suspect that doesn't matter.

Thanks!
--rafal