y2038: make ns_to_compat_timeval use __kernel_old_timeval

This gets us one step closer to removing 'struct timeval' from the
kernel. We still keep __kernel_old_timeval for interfaces that we cannot
fix otherwise, and ns_to_compat_timeval() is provably safe for interfaces
that are legitimate users of __kernel_old_timeval on native kernels,
so this is an obvious change.

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