some swapper problems analogous to those experienced with ccd.
This fix is a kludge: since we currently don't track the "sector size"
in a volume label, we guess a worst case (4 kB, as used by vnode
devices). If the concept of sector size is here to stay, I'll make
some changes to track the "sector size" of a volume. This will
probably be the maximum of the sector sizes of all component drives,
but things could get ugly if we start allowing non-standard sector
sizes such as 524 bytes.
Unkludged-version-submitted-by: phk
03705cdeb7