tick/sched: Optimize tick_do_update_jiffies64() further

[ Upstream commit 7a35bf2a ]

Now that it's clear that there is always one tick to account, simplify the
calculations some more.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20201117132006.565663056@linutronix.de


Stable-dep-of: e9523a0d ("tick/common: Align tick period with the HZ tick.")
Signed-off-by: Sasha Levin <sashal@kernel.org>