softupdates panics fixed.

I found the bug that I introduced around the 29th of august.  It is fixed
in ffs_softdep.c rev 1.143.  Truely, most of the leg work was done by
tegge.  I just produced and tested a patch.  This completes a buildworld
with 128M of memory now, whereas before it just completed with 64m and
512m.  perhaps only by chance.

Cheers,
Jeff