imsm: Set disk slot number

imsm: Set disk slot number

If first disk of IMSM RAID1 is failed but still present in the system,
the array is not auto-assembled. Auto-assemble uses raid disk slot from
metadata to index disks. As it's not set, the valid disk is seen as a
replacement disk and its metadata is ignored. The problem is not
observed for other RAID levels as they have more than 2 disks -
replacement disks are only stored under uneven indexes so third disk
metadata is used in such scenario.

Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@intel.com>
Reviewed-by: Tomasz Majchrzak <tomasz.majchrzak@intel.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>