mm,page_owner: maintain own list of stack_records structs

page_owner needs to increment a stack_record refcount when a new
allocation occurs, and decrement it on a free operation.  In order to do
that, we need to have a way to get a stack_record from a handle. 
Implement __stack_depot_get_stack_record() which just does that, and make
it public so page_owner can use it.

Also, traversing all stackdepot buckets comes with its own complexity,
plus we would have to implement a way to mark only those stack_records
that were originated from page_owner, as those are the ones we are
interested in.  For that reason, page_owner maintains its own list of
stack_records, because traversing that list is faster than traversing all
buckets while keeping at the same time a low complexity.

For now, add to stack_list only the stack_records of dummy_handle and
failure_handle, and set their refcount of 1.

Further patches will add code to increment or decrement stack_records
count on allocation and free operation.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Oscar Salvador <[email protected]>
Reviewed-by: Vlastimil Babka <[email protected]>
Reviewed-by: Marco Elver <[email protected]>
Acked-by: Andrey Konovalov <[email protected]>
Cc: Alexander Potapenko <[email protected]>
Cc: Michal Hocko <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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