cpupool: Avoid race when moving cpu between cpupools

cpupool: Avoid race when moving cpu between cpupools

Moving cpus between cpupools is done under the schedule lock of the
moved cpu.  When checking a cpu being member of a cpupool this must be
done with the lock of that cpu being held.  Hot-unplugging of physical
cpus might encounter the same problems, but this should happen only
very rarely.

Signed-off-by: Juergen Gross <juergen.gross@ts.fujitsu.com>
Acked-by: Andre Przywara <andre.przywara@amd.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>