fix remote degraded gmirror

[Robin Becker]

mojo fms wrote:
.......
 >
 >
 >  What about shrinking the old mirror drive a few megs so its smaller than
 > the new one?
 >
The original problem has gone away for the moment as my hoter found an AAJS 
drive with the same number of sectors and the mirror synchronized fine.

I looked around for ways to shrink the slice, but didn't discover anything very 
authoritative or easy. Is there a slice reduction beast?
-- 
Robin Becker