jump_label: Simplify and clarify static_key_fast_inc_cpus_locked()

[ Upstream commit 9bc2ff87 ]

Make the code more obvious and add proper comments to avoid future head
scratching.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20240610124406.548322963@linutronix.de


Stable-dep-of: 1d7f856c ("jump_label: Fix static_key_slow_dec() yet again")
Signed-off-by: Sasha Levin <sashal@kernel.org>