y2038: vdso: change timespec to __kernel_old_timespec

In order to remove 'timespec' completely from the kernel, all
internal uses should be converted to a y2038-safe type, while
those that are only for compatibity with existing user space
should be marked appropriately.

Change vdso to use __kernel_old_timespec in order to avoid
the deprecated type and mark these interfaces as outdated.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>