xen: credit1: treat pCPUs more evenly during balancing.

xen: credit1: treat pCPUs more evenly during balancing.

Right now, we use cpumask_first() for going through
the bus pCPUs in csched_load_balance(). This means
not all pCPUs have equal chances of seeing their
pending work stolen. It also means there is more
runqueue lock pressure on lower ID pCPUs.

To avoid all this, let's record and remember, for
each NUMA node, from what pCPU we have stolen for
last, and start from that the following time.

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