login: Disarm timer after utmp lock acquisition [BZ #24879]

If the file processing takes a long time for some reason, SIGALRM can
arrive while the file is still being processed.  At that point, file
access will fail with EINTR.  Disarming the timer after lock
acquisition avoids that.  (If there was a previous alarm, it is the
responsibility of the caller to deal with the EINTR error.)