security: keys: Replace time_t with time64_t for struct key_preparsed_payload

The 'struct key_preparsed_payload' will use 'time_t' which we will
try to remove in the kernel, since 'time_t' is not year 2038 safe on
32bits systems.

Thus this patch replaces 'time_t' with 'time64_t' which is year 2038
safe on 32 bits system for 'struct key_preparsed_payload', moreover
we should use the 'TIME64_MAX' macro to initialize the 'time64_t'
type variable.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: James Morris <james.l.morris@oracle.com>