md: fix buglet in RAID5 -> RAID0 conversion.

RAID5 uses a 'per-array' value for the 'size' of each device.
RAID0 uses a 'per-device' value - it can be different for each device.

When converting a RAID5 to a RAID0 we must ensure that the per-device
size of each device matches the per-array size for the RAID5, else
the array will change size.

If the metadata cannot record a changed per-device size (as is the
case with v0.90 metadata) the array could get bigger on restart. This
does not cause data corruption, so it not a big issue and is mainly
yet another a reason to not use 0.90.

Signed-off-by: NeilBrown <[e-mail]>

drivers/md/raid0.c

···
 			       mdname(mddev));
 			return ERR_PTR(-EINVAL);
 		}
+		rdev->sectors = mddev->dev_sectors;
 	}
 
 	/* Set new parameters */