imsm: robustify recovery-start detection

imsm: robustify recovery-start detection

update_recovery_start() assumed that the out-of-sync disk would always be
marked as IMSM_ORD_REBUILD in the disk_ord_tbl, but the segmentation
fault reported by Andy proves otherwise.  This might also be explained by
an interrupted rebuild and the disk has not yet been marked missing.

https://bugzilla.redhat.com/show_bug.cgi?id=592030

Reported-by: Andy Lutomirski <luto@mit.edu>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>