graid: unbreak Promise RAID1 with 4+ providers

Fix a problem in graid implementation of Promise RAID1 created with 4+ disks.
Such an array generally works fine until reboot only due to a bug
in metadata writing code. Before the fix, next taste erronously created
RAID1E (kind of RAID10) instead of RAID1, hence graid used wrong offsets
for I/O operations.

The bug did not affect Promise RAID1 arrays with 2 or 3 disks only.

Reviewed by: mav
MFC after: 3 days