MFC r337972: 9751 Allocation throttling misplacing ditto blocks

Relax allocation throttling for ditto blocks. Due to random imbalances
in allocation it tends to push block copies to one vdev, that looks
slightly better at the moment. Slightly less strict policy allows both
improve data security and surprisingly write performance, since we don't
need to touch extra metaslabs on each vdev to respect the min distance.

Sponsored by: iXsystems, Inc.