imsm: fix mark_failure / introduce mark_missing

imsm: fix mark_failure / introduce mark_missing

Actually, rename mark_failure to mark_missing and then implement the
correct mark_failure which according to new documentation is to:

1/ Set the FAILED status bit
2/ Set IMSM_ORD_REBUILD to mark the disk out of sync
3/ Set map->failed_disk_num if this is the first failure detected
   failure (it is ~0 otherwise)

Previously the assumption was that IMSM_ORD_REBUILD only appeared in
map[1], so all routines that care about out-of-sync disks need to be
updated.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>