staging/lustre: use 64-bit times for ptlrpc sec expiry

The exp_flvr_expire and imp_sec_expire are defined as 'unsigned long',
which doesn't overflow until 2106, but to be on the safe side, this
changes the code to use time64_t like we do everywhere else.

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>