dnode_special_open
dnode_special_open(os, &os->os_phys->os_meta_dnode,
dnode_special_open(os, &os->os_phys->os_userused_dnode,
dnode_special_open(os, &os->os_phys->os_groupused_dnode,
dnode_special_open(os,
void dnode_special_open(struct objset *dd, dnode_phys_t *dnp,