staging/lustre: use 64-bit time for adaptive timeout

The adaptive timeout handling stores absolute times in 32-bit time_t
quantities, which will overflow in 2038.

This changes it to use time64_t.

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>