OpenZFS 9682 - page fault in dsl_async_clone_destroy() while opening pool

OpenZFS 9682 - page fault in dsl_async_clone_destroy() while opening pool

Authored by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed by: Brad Lewis <brad.lewis@delphix.com>
Reviewed by: Matt Ahrens <matt@delphix.com>
Reviewed by: Sara Hartse <sara.hartse@delphix.com>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Approved by: Robert Mustacchi <rm@joyent.com>
Ported-by: George Melikov <mail@gmelikov.ru>
OpenZFS-issue: https://www.illumos.org/issues/9682
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/ade2c82828
Closes #8037