When your device count falls below your replica count you can either add
devices or reduce the replica count.
Trying to reduce your replica count fails about half the time because
removing parts from from failed devices temporarily invalidates your
_replica2part2dev table with NONE_DEV which can result in an IndexError
in _adjust_replica2part2dev_size.
If you adjust the replica count first you won't have to worry about
tracking unassigned parts from failed devices.
Closes-Bug: #1558751
Change-Id: I99dc776fd260a2ba68ca77d7b5ed5120d10b06de