imsm: allocate buffer to support maximum sector size

imsm: allocate buffer to support maximum sector size

Allocate migration record buffer to support maximum sector size. Disk with
non-matching sector size is not going to be included in the array, however
some preparation/cleanup actions still take place on it and they would
cause a crash. Clear migration record using sector size of the disk (not
array) as they might not match.

Signed-off-by: Alexey Obitotskiy <aleksey.obitotskiy@intel.com>
Signed-off-by: Tomasz Majchrzak <tomasz.majchrzak@intel.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>