mm: memcontrol: make the slab calculation consistent

Although the ratio of the slab is one, we also should read the ratio from
the related memory_stats instead of hard-coding.  And the local variable
of size is already the value of slab_unreclaimable.  So we do not need to
read again.

To do this we need some code like below:

if (unlikely(memory_stats[i].idx == NR_SLAB_UNRECLAIMABLE_B)) {
-	size = memcg_page_state(memcg, NR_SLAB_RECLAIMABLE_B) +
-	       memcg_page_state(memcg, NR_SLAB_UNRECLAIMABLE_B);
+       VM_BUG_ON(i < 1);
+       VM_BUG_ON(memory_stats[i - 1].idx != NR_SLAB_RECLAIMABLE_B);
+	size += memcg_page_state(memcg, memory_stats[i - 1].idx) *
+		memory_stats[i - 1].ratio;

It requires a series of VM_BUG_ONs or comments to ensure these two items
are actually adjacent and in the right order.  So it would probably be
easier to implement this using a wrapper that has a big switch() for unit
conversion.

More details about this discussion can refer to:

    https://lore.kernel.org/patchwork/patch/1348611/

This would fix the ratio inconsistency and get rid of the order
guarantee.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Muchun Song <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Cc: Feng Tang <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: NeilBrown <[email protected]>
Cc: Pankaj Gupta <[email protected]>
Cc: Rafael. J. Wysocki <[email protected]>
Cc: Randy Dunlap <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Sami Tolvanen <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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