xen: sched: impove use of cpumask scratch space in Credit1.

xen: sched: impove use of cpumask scratch space in Credit1.

It is ok to use just cpumask_scratch in csched_runq_steal().
In fact, the cpu parameter comes from the cpu local variable
in csched_load_balance(), which in turn comes from cpu in
csched_schedule(), which is smp_processor_id().

While there, also:
 - move the comment about cpumask_scratch in the header
   where the scratch space is declared;
 - spell more clearly (in that same comment) what are the
   serialization rules.

No functional change intended.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>