[[https://github.com/illumos/illumos-gate/commit/1271e4b10dfaaed576c08a812f466f6e81370e5e]]
[[https://www.illumos.org/issues/8585]]
[[https://github.com/openzfs/openzfs/pull/447]]
This review request is to invite reviewers and testers for this rather complex change.
Also, please note that this MFV reverts FreeBSD commit rS314549 to make the merge easier.
Another note is that at present our emulation of `cv_timedwait_hires` is rather poor, so I elected to use `cv_timedwait_sbt` directly.