graid: MFC: unbreak Promise RAID1 with 4+ providers

graid: MFC: 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
Approved-by:	re (cperciva)

(cherry picked from commit 81092e92ea5184c4eeedad58044d72cfef72dd24)
(cherry picked from commit 77814c95994aab102be0e327de788dc1c00c023d)