Without a subsequent wbinvd the changes to suspended_cpus (and
resuming_cpus) can be lost at least on AMD systems that use MOESI cache
coherency protocol. That can happen because one of APs ends up as an
Owner of the corresponding cache line(s) and the changes may never reach
the main memory before the AP is reset.
While here, move clearing of suspended_cpus a little bit earlier. Also,
rework the comment that describes the need for resuming_cpus.