vn_open_downgrade
vn_open_downgrade(cs->vp, fflags);
vn_open_downgrade(fp->rf_vp, FREAD);
vn_open_downgrade(fp->rf_vp, FREAD|FWRITE);
vn_open_downgrade(vp, FREAD);
vn_open_downgrade(vp, FREAD|FWRITE);
void vn_open_downgrade(vnode_t *, int);