USER_ONE was only ensuring id->us was non-zero, while USER_REAL ensures it is non-zero **and** >1 in case fake_user flag is set (which at least ensures us unsetting fake_user won't leave id->us in invalid zero state).
USER_ONE was only ensuring id->us was non-zero, while USER_REAL ensures it is non-zero **and** >1 in case fake_user flag is set (which at least ensures us unsetting fake_user won't leave id->us in invalid zero state).