staging/lustre: use time64_t for l_last_activity

The l_last_activity struct member is used to keep track lock hold
times, and it is printed for debugging purposes. For the elapsed
time, we can use 'long' here, but it's better to use time64_t
for storing the real time to avoid an overflow in 2038.

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>