9485 Optimize possible split block search space
illumos/illumos-gate@a21fe349793c3805ec504bbe5e9acf06c2d63d7a
https://github.com/illumos/illumos-gate/commit/a21fe349793c3805ec504bbe5e9acf06c2d63d7a
https://www.illumos.org/issues/9485
Port this commit from ZoL: https://github.com/zfsonlinux/zfs/commit/4589f3ae4c1bb435777da8640eb915f3c713b14d
Author: Brian Behlendorf <behlendorf1@llnl.gov>