mm: Scrub pages in alloc_heap_pages() if needed

mm: Scrub pages in alloc_heap_pages() if needed

When allocating pages in alloc_heap_pages() first look for clean pages. If none
is found then retry, take pages marked as unscrubbed and scrub them.

Note that we shouldn't find unscrubbed pages in alloc_heap_pages() yet. However,
this will become possible when we stop scrubbing from free_heap_pages() and
instead do it from idle loop.

Since not all allocations require clean pages (such as xenheap allocations)
introduce MEMF_no_scrub flag that callers can set if they are willing to
consume unscrubbed pages.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>