ztest: ENOSPC in ztest_objset_destroy_cb()

While unlikely it is possible for dsl_destroy_head() to return
ENOSPC in the ztest_objset_destroy_cb(). This can occur even
when ZFS_SPACE_CHECK_DESTROY is used with the dsl_sync_task().
Both the existence of a checkpoint and pending deferred frees
can cause this.

Reviewed-by: Serapheim Dimitropoulos <serapheim.dimitro@delphix.com>
Reviewed-by: Tom Caputi <tcaputi@datto.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8206