mm: rename page_counter's count/limit into usage/max

This patch renames struct page_counter fields:
  count -> usage
  limit -> max

and the corresponding functions:
  page_counter_limit() -> page_counter_set_max()
  mem_cgroup_get_limit() -> mem_cgroup_get_max()
  mem_cgroup_resize_limit() -> mem_cgroup_resize_max()
  memcg_update_kmem_limit() -> memcg_update_kmem_max()
  memcg_update_tcp_limit() -> memcg_update_tcp_max()

The idea behind this renaming is to have the direct matching
between memory cgroup knobs (low, high, max) and page_counters API.

This is pure renaming, this patch doesn't bring any functional change.

Link: http://lkml.kernel.org/r/20180405185921.4942-1-guro@fb.com
Signed-off-by: Roman Gushchin <guro@fb.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 517096c..577a19a 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -467,7 +467,7 @@ unsigned long mem_cgroup_get_zone_lru_size(struct lruvec *lruvec,
 
 void mem_cgroup_handle_over_high(void);
 
-unsigned long mem_cgroup_get_limit(struct mem_cgroup *memcg);
+unsigned long mem_cgroup_get_max(struct mem_cgroup *memcg);
 
 void mem_cgroup_print_oom_info(struct mem_cgroup *memcg,
 				struct task_struct *p);
@@ -858,7 +858,7 @@ mem_cgroup_node_nr_lru_pages(struct mem_cgroup *memcg,
 	return 0;
 }
 
-static inline unsigned long mem_cgroup_get_limit(struct mem_cgroup *memcg)
+static inline unsigned long mem_cgroup_get_max(struct mem_cgroup *memcg)
 {
 	return 0;
 }