login: Remove double-assignment of fl.l_whence in try_file_lock

Since l_whence is the second member of struct flock, it is written
twice.  The double-assignment is technically undefined behavior due to
the lack of a sequence point.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Change-Id: I2baf9e70690e723c61051b25ccbd510aec15976c
(cherry picked from commit b0a83ae7)