y2038: socket: remove timespec reference in timestamping

In order to remove the 'struct timespec' definition and the
timespec64_to_timespec() helper function, change over the in-kernel
definition of 'struct scm_timestamping' to use the __kernel_old_timespec
replacement and open-code the assignment.

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