12002 async unlinked drain

Review Request #2461 — Created Nov. 22, 2019 and submitted — Latest diff uploaded


This is a port from ZoL which was in turn a port from Nexenta.

The ZoL code added a new test case (umount_unlinked_drain) but that code currently depends on Linux /proc and also other kstat changes which we don't yet have in illumos. I am delivering that test but I did not add it to the run files.

The original Nexenta code also added a new test case for this change (umount_002). I have also included that test case since it does run on illumos and I have added that to the run files.