y2038: add __kernel_old_timespec and __kernel_old_time_t

The 'struct timespec' definition can no longer be part of the uapi headers
because it conflicts with a a now incompatible libc definition. Also,
we really want to remove it in order to prevent new uses from creeping in.

The same namespace conflict exists with time_t, which should also be
removed. __kernel_time_t could be used safely, but adding 'old' in the
name makes it clearer that this should not be used for new interfaces.

Add a replacement __kernel_old_timespec structure and __kernel_old_time_t
along the lines of __kernel_old_timeval.

Acked-by: Deepa Dinamani <deepa.kernel@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>