mm: Keep heap accessible to others while scrubbing

mm: Keep heap accessible to others while scrubbing

Instead of scrubbing pages while holding heap lock we can mark
buddy's head as being scrubbed and drop the lock temporarily.
If someone (most likely alloc_heap_pages()) tries to access
this chunk it will signal the scrubber to abort scrub by setting
head's BUDDY_SCRUB_ABORT bit. The scrubber checks this bit after
processing each page and stops its work as soon as it sees it.

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