Incremental: honor an 'enough' flag from external handlers

Incremental: honor an 'enough' flag from external handlers

This is needed for imsm where:
1/ we want to report raid_disks as zero to allow mdadm -As to
   incorporate all spares
2/ we can't determine stale disks by looking at the event counts.
3/ we can't see per-subarray expectations with the info returned from
   the container level ->getinfo_super()

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