mm: initialize lowmem virq when boot-time scrubbing is disabled

mm: initialize lowmem virq when boot-time scrubbing is disabled

scrub_heap_pages() does early return if boot-time scrubbing is
disabled, neglecting to initialize lowmem VIRQ.

Because setup_low_mem_virq() doesn't logically belong in
scrub_heap_pages() we put them both into the newly added
heap_init_late().

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>