login: Acquire write lock early in pututline [BZ #24882]

It has been reported that due to lack of fairness in POSIX file
locking, the current reader-to-writer lock upgrade can result in
lack of forward progress.  Acquiring the write lock directly
hopefully avoids this issue if there are only writers.

This also fixes bug 24882 due to the cache revalidation in
__libc_pututline.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Change-Id: I57e31ae30719e609a53505a0924dda101d46372e
(cherry picked from commit be6b16d9)