libceph: use timespec64 for r_mtime

The request mtime field is used all over ceph, and is currently
represented as a 'timespec' structure in Linux. This changes it to
timespec64 to allow times beyond 2038, modifying all users at the
same time.

[ Remove now redundant ts variable in writepage_nounlock(). ]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>