y2038: rename itimerval to __kernel_old_itimerval

Take the renaming of timeval and timespec one level further,
also renaming itimerval to __kernel_old_itimerval, to avoid
namespace conflicts with the user-space structure that may
use 64-bit time_t members.

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