mm/memcontrol.c: make mem_cgroup_id_get_many() __maybe_unused

mem_cgroup_id_get_many() is currently used only when MMU or MEMCG_SWAP
configuration options are enabled.  Having them disabled triggers the
following warning at compile time:

  linux/mm/memcontrol.c:4797:13: warning: `mem_cgroup_id_get_many' defined but not used [-Wunused-function]
   static void mem_cgroup_id_get_many(struct mem_cgroup *memcg, unsigned int n)

Make mem_cgroup_id_get_many() __maybe_unused to address the issue.
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Chris Down <chris@chrisdown.name>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Link: http://lkml.kernel.org/r/20200305164354.48147-1-vincenzo.frascino@arm.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -4863,7 +4863,8 @@ static void mem_cgroup_id_remove(struct mem_cgroup *memcg)
 	}
 }
 
-static void mem_cgroup_id_get_many(struct mem_cgroup *memcg, unsigned int n)
+static void __maybe_unused mem_cgroup_id_get_many(struct mem_cgroup *memcg,
+						  unsigned int n)
 {
 	refcount_add(n, &memcg->id.ref);
 }