staging/lustre: use 64-bit times for ptlrpc_sec

Here we use an unsigned long to store the timeout for gc,
which is probably safe until 2106, but this patch converts it
to use ktime_get_real_seconds() and time64_t for consistency.

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>