xen/sched: fix locking in restore_vcpu_affinity()

xen/sched: fix locking in restore_vcpu_affinity()

Commit 0763cd2687897b55e7 ("xen/sched: don't disable scheduler on cpus
during suspend") removed a lock in restore_vcpu_affinity() which needs
to stay: cpumask_scratch_cpu() must be protected by the scheduler
lock. restore_vcpu_affinity() is being called by thaw_domains(), so
with multiple domains in the system another domain might already be
running and the scheduler might make use of cpumask_scratch_cpu()
already.

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