rrm_enter
void rrm_enter(rrmlock_t *rrl, krw_t rw, void *tag);
rrm_enter(&(*zfvp)->z_teardown_lock, (writer) ? RW_WRITER :
rrm_enter(&zfsvfs->z_teardown_lock, RW_WRITER, FTAG);
rrm_enter(&zfsvfs->z_teardown_lock, RW_READER, FTAG);