staging/lustre: partially use time64_t for capa expiry

Here, we actually have a problem in 2038 or at the latest in 2106 when
the lc_expiry variable on the wire protocol overflows, including on
64-bit architectures.

Now that the actual code was removed, only on-the wire structure
is left so we add in a y2038 warning to remind whoever gets to
implement it again about this problem.

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>