timer: Prepare to change all DEFINE_TIMER() callbacks

Before we can globally change the function prototype of all timer callbacks,
we have to change those set up by DEFINE_TIMER(). Prepare for this by
casting the callbacks until the prototype changes globally.

Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Kees Cook <keescook@chromium.org>