staging/lustre: use 64-bit time for ni_last_alive

The ni_last_alive member of lnet_ni uses a 'long' to store a timestamp,
which breaks on 32-bit systems in 2038.

This changes it to use time64_t and the respective functions for it.

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>