mm: Place unscrubbed pages at the end of pagelist

mm: Place unscrubbed pages at the end of pagelist

.. so that it's easy to find pages that need to be scrubbed (those pages are
now marked with _PGC_need_scrub bit).

We keep track of the first unscrubbed page in a page buddy using first_dirty
field. For now it can have two values, 0 (whole buddy needs scrubbing) or
INVALID_DIRTY_IDX (the buddy does not need to be scrubbed). Subsequent patches
will allow scrubbing to be interrupted, resulting in first_dirty taking any
value.

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