y2038: socket: use __kernel_old_timespec instead of timespec

The 'timespec' type definition and helpers like ktime_to_timespec()
or timespec64_to_timespec() should no longer be used in the kernel so
we can remove them and avoid introducing y2038 issues in new code.

Change the socket code that needs to pass a timespec to user space for
backward compatibility to use __kernel_old_timespec instead.  This type
has the same layout but with a clearer defined name.

Slightly reformat tcp_recv_timestamp() for consistency after the removal
of timespec64_to_timespec().

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