Use disk sector size value to set offset for reading GPT

Use disk sector size value to set offset for reading GPT

mdadm is using invalid byte-offset while reading GPT header to get
partition info (size, first sector, last sector etc.). Now this offset
is hardcoded to 512 bytes and it is not valid for disks with sector
size different than 512 bytes because MBR and GPT headers are aligned
to LBA, so valid offset for 4k drives is 4096 bytes.

Signed-off-by: Mariusz Dabrowski <mariusz.dabrowski@intel.com>
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>