Если кому нужно - вот примитивный патч, исправляющий kernel panic:

cat geom_vfs.c.diff
--- geom_vfs.c.orig     2007-12-29 11:48:20.000000000 +0000
+++ geom_vfs.c  2007-12-29 11:48:39.000000000 +0000
@@ -71,16 +71,9 @@
        struct buf *bp;
        int vfslocked;

-       if (bip->bio_error) {
-               printf("g_vfs_done():");
-               g_print_bio(bip);
-               printf("error = %d\n", bip->bio_error);
-       }
        bp = bip->bio_caller2;
        bp->b_error = bip->bio_error;
        bp->b_ioflags = bip->bio_flags;
-       if (bip->bio_error)
-               bp->b_ioflags |= BIO_ERROR;
        bp->b_resid = bp->b_bcount - bip->bio_completed;
        g_destroy_bio(bip);
        vfslocked = VFS_LOCK_GIANT(((struct mount *)NULL));

Применяем так:

%cd /usr/src/sys/geom/
%cat geom_vfs.c.diff | patch -p1