OpenZFS 7028 - avl_destroy_nodes supports emptying, not just destroying, an avl tree

OpenZFS 7028 - avl_destroy_nodes supports emptying, not just destroying, an avl tree

Authored by: Eli Rosenthal <eli.rosenthal@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Igor Kozhukhov <ikozhukhov@gmail.com>
Approved by: Gordon Ross <gordon.ross@nexenta.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>
OpenZFS-issue: https://www.illumos.org/issues/7028
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/86f617e
Closes #6583