10844 Serialize ZTHR operations to eliminate races

illumos/illumos-gate@6a316e1f6d32750bb8fcf2558dcb17b90ca580fd
https://github.com/illumos/illumos-gate/commit/6a316e1f6d32750bb8fcf2558dcb17b90ca580fd

https://www.illumos.org/issues/10844

ZoL 61c3391acc9 Serialize ZTHR operations to eliminate races

Portions contributed by: Jerry Jelinek <jerry.jelinek@joyent.com>
Author: Serapheim Dimitropoulos <serapheim@delphix.com>