xen/sched: fix credit2 smt idle handling

xen/sched: fix credit2 smt idle handling

Credit2's smt_idle_mask_set() and smt_idle_mask_clear() are used to
identify idle cores where vcpus can be moved to. A core is thought to
be idle when all siblings are known to have the idle vcpu running on
them.

Unfortunately the information of a vcpu running on a cpu is per
runqueue. So in case not all siblings are in the same runqueue a core
will never be regarded to be idle, as the sibling not in the runqueue
is never known to run the idle vcpu.

Use a credit2 specific cpumask of siblings with only those cpus
being marked which are in the same runqueue as the cpu in question.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Dario Faggioli <dfaggioli@suse.com>