[Patch mdadm-3.2.2] Fix the fix for the readd bug

The readd bug fix didn't work due to a faulty test.  We need to check raid_disks to get an upper bound of the found slots to check since nr_disks only counts good disks and we could have faulty or spares that aren't counted but amount to an occupied slot.

Signed-off-by: Doug Ledford <redacted>