zthr_create
arc_adjust_zthr = zthr_create(arc_adjust_cb_check,
zthr_create(spa_checkpoint_discard_thread_check,
extern zthr_t *zthr_create(zthr_checkfunc_t checkfunc,
spa->spa_condense_zthr = zthr_create(spa_condense_indirect_thread_check,