staging/lustre: use 64-bit time for selftest

The lustre selftest code has multiple time stamps that are kept
as 'time_t' or 'unsigned long' and can therefore overflow on
32-bit systems.

This changes the code to use time64_t instead.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>