mm: Scrub memory from idle loop

mm: Scrub memory from idle loop

Instead of scrubbing pages during guest destruction (from
free_heap_pages()) do this opportunistically, from the idle loop.

We might come to scrub_free_pages()from idle loop while another CPU
uses mapcache override, resulting in a fault while trying to do
__map_domain_page() in scrub_one_page(). To avoid this, make mapcache
vcpu override a per-cpu variable.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Julien Grall <julien.grall@arm.com>