imsm: fix reshape for >2TB drives

imsm: fix reshape for >2TB drives

If reshape is performed on drives larger then 2 TB,
migration checkpoint area that is calculated exeeds 32-bit value.
This checkpoint area is a reserved space threated as backup
during reshape - at the end of the drive, right before metadata.
As a result - wrong space is used and the data that may exists there
is overwritten.

Adding additional field to migration record to track high order 32-bits
of pba of this area. Three other fields that may exceed 32-bit value
for large drives are added as well.

Signed-off-by: Pawel Baldysiak <pawel.baldysiak@intel.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>