mm/memcg: minor cleanup for MEM_CGROUP_ID_MAX

MEM_CGROUP_ID_MAX is only used when CONFIG_MEMCG is configured.  So remove
unneeded !CONFIG_MEMCG variant.  Also it's only used in
mem_cgroup_alloc(), so move it from memcontrol.h to memcontrol.c.  And
further define it as:

  #define MEM_CGROUP_ID_MAX ((1UL << MEM_CGROUP_ID_SHIFT) - 1)

so if someone changes MEM_CGROUP_ID_SHIFT in the future, then
MEM_CGROUP_ID_MAX will be updated accordingly, as suggested by Muchun.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Miaohe Lin <[email protected]>
Reviewed-by: Muchun Song <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Shakeel Butt <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: MiaoheLin

include/linux/memcontrol.h

@@ -61,7 +61,6 @@ struct mem_cgroup_reclaim_cookie {
 #ifdef CONFIG_MEMCG
 
 #define MEM_CGROUP_ID_SHIFT	16
-#define MEM_CGROUP_ID_MAX	USHRT_MAX
 
 struct mem_cgroup_id {
 	int id;
@@ -1158,7 +1157,6 @@ unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t *pgdat, int order,
 #else /* CONFIG_MEMCG */
 
 #define MEM_CGROUP_ID_SHIFT	0
-#define MEM_CGROUP_ID_MAX	0
 
 static inline struct mem_cgroup *folio_memcg(struct folio *folio)
 {

mm/memcontrol.c

@@ -5155,6 +5155,7 @@ static struct cftype mem_cgroup_legacy_files[] = {
  * those references are manageable from userspace.
  */
 
+#define MEM_CGROUP_ID_MAX	((1UL << MEM_CGROUP_ID_SHIFT) - 1)
 static DEFINE_IDR(mem_cgroup_idr);
 
 static void mem_cgroup_id_remove(struct mem_cgroup *memcg)