jump_label: Split out code under the hotplug lock

In order to later introduce an "already locked" version of some
of the static key funcions, let's split the code into the core stuff
(the *_cpuslocked functions) and the usual helpers, which now
take/release the hotplug lock and call into the _cpuslocked
versions.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-arm-kernel@lists.infradead.org
Link: http://lkml.kernel.org/r/20170801080257.5056-3-marc.zyngier@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>