y2038: move itimer reset into itimer.c

Preparing for a change to the itimer internals, stop using the
do_setitimer() symbol and instead use a new higher-level interface.

The do_getitimer()/do_setitimer functions can now be made static,
allowing the compiler to potentially produce better object code.

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>