OpenZFS 7253 - ztest failure: dsl_destroy_head(name) == 0 (0x10 == 0x0), file .....

OpenZFS 7253 - ztest failure: dsl_destroy_head(name) == 0 (0x10 == 0x0), file ../ztest.c, line 3235 (#5660)

Authored by: Chris Williamson <chris.williamson@delphix.com>
Reviewed by: - Matthew Ahrens <mahrens@delphix.com>
Reviewed by: - Paul Dagnelie <pcd@delphix.com>
Approved by: - Robert Mustacchi <rm@joyent.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>
OpenZFS-issue: https://www.illumos.org/issues/7253
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/754998c
Closes #5660