posix-timers: Clear overrun in common_timer_set()

Keeping the overrun count of the previous setup around is just wrong. The
new setting has nothing to do with the previous one and has to start from a
clean slate.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>