sched: clarify use cases of schedule_cpu_switch()

sched: clarify use cases of schedule_cpu_switch()

schedule_cpu_switch() is meant to be only used for moving
pCPUs from a cpupool to no cpupool, and from there back
to a cpupool, *not* to move them directly from one cpupool
to another.

This is something inherent to the way the function is
implemented and called, but is not that clear, just by the
look of it.

Make it more evident by:
 - adding commentary and ASSERT()s;
 - update the cpupool per-CPU variable (mapping pCPUs to
   pools) directly in schedule_cpu_switch(), rather than
   in various places in cpupool.c.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Juergen Gross <jgross@suse.com>
Reviewed-by: George Dunlap <george.dunlap@eu.citrix.com>