OpenZFS 6871 - libzpool implementation of thread_create should enforce length is 0

OpenZFS 6871 - libzpool implementation of thread_create should enforce length is 0

Porting notes:
- Several direct callers of zk_thread_create() are passing TS_RUN for the
length.  The `len` and `state` were inverted,this commit fixes them.

Authored by: Eli Rosenthal <eli.rosenthal@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: George Melikov mail@gmelikov.ru
OpenZFS-issue: https://www.illumos.org/issues/6871
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/8fc9228
Closes #5621