lib/on-cpus: Correct and simplify synchronization

get/put_on_cpu_info() were providing per-cpu locking for the per-cpu
on_cpu info, but it's difficult to reason that they're correct since
they use test_and_set/clear rather than a typical lock. Just revert
to a typical spinlock to simplify it. Also simplify the break case
for on_cpu_async() - we don't care if func is NULL, we only care
that the cpu is idle. And, finally, add a missing barrier to
on_cpu_async(). Before commit 01855004 ("arm/arm64: Remove
spinlocks from on_cpu_async") the spin_unlock() provided an implicit
barrier at the correct location, but moving the release to the more
logical location, below the setting of idle, lost it.

Fixes: 01855004 ("arm/arm64: Remove spinlocks from on_cpu_async")
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Andrew Jones <andrew.jones@linux.dev>