Repair the termination condition of a write(2) loop.

Repair the termination condition of a write(2) loop.

Since _PATH_MASTERPASSWD_LOCK is on a local file system in any sane setup
and written to in blocking mode, i don't see how write(2) could return
before having written everything, so this maybe wasn't an actual bug,
but it should be repaired anyway, if only for clarity and extra safety.

From Ben Cornett <ben at lantern dot is>;
ok millert@.