y2038: Change sys_utimensat() to use __kernel_timespec

When 32-bit architectures get changed to support 64-bit time_t,
utimensat() needs to use the new __kernel_timespec structure as its
argument.

The older utime(), utimes() and futimesat() system calls don't need a
corresponding change as they are no longer used on C libraries that have
64-bit time support.

As we do for the other syscalls that have timespec arguments, we reuse
the 'compat' syscall entry points to implement the traditional four
interfaces, and only leave the new utimensat() as a native handler,
so that the same code gets used on both 32-bit and 64-bit kernels
on each syscall.

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