Illumos 5347 - idle pool may run itself out of space

Illumos 5347 - idle pool may run itself out of space

5347 idle pool may run itself out of space
Reviewed by: Alex Reece <alex.reece@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Steven Hartland <killing@multiplay.co.uk>
Reviewed by: Richard Elling <richard.elling@richardelling.com>
Approved by: Dan McDonald <danmcd@omniti.com>

References:
  https://github.com/illumos/illumos-gate/commit/231aab8
  https://github.com/illumos/illumos-gate/commit/4a92375 3642
  https://www.illumos.org/issues/5347
  https://github.com/zfsonlinux/zfs/commit/89b1cd6 (partial commit & fix)
  https://github.com/zfsonlinux/zfs/commit/fbeddd6 Illumos 4390
  https://github.com/zfsonlinux/zfs/commit/2696dfa Illumos 3642, 3643

Porting notes:
This is completing the partial fix from FreeBSD

Ported-by: kernelOfTruth kerneloftruth@gmail.com
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3586