timekeeping: Consolidate fast timekeeper

[ Upstream commit 90be8d6c ]

Provide a inline function which replaces the copy & pasta.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220415091921.072296632@linutronix.de


Stable-dep-of: 5c1806c4 ("kcsan, seqlock: Support seqcount_latch_t")
Signed-off-by: Sasha Levin <sashal@kernel.org>