mm: simplify folio_memcg_charged()

There's no need to check which kind of pointer is in the memcg_data field,
all we actually care about is whether it's zero or not.  Saves 70 bytes in
workingset_activation() with the Debian config.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Acked-by: Shakeel Butt <[email protected]>
Acked-by: Roman Gushchin <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Zi Yan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Matthew Wilcox (Oracle)

include/linux/memcontrol.h

@@ -438,9 +438,7 @@ static inline struct mem_cgroup *folio_memcg(struct folio *folio)
  */
 static inline bool folio_memcg_charged(struct folio *folio)
 {
-	if (folio_memcg_kmem(folio))
-		return __folio_objcg(folio) != NULL;
-	return __folio_memcg(folio) != NULL;
+	return folio->memcg_data != 0;
 }
 
 /*