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/mm/hugetlb_cgroup.c b/mm/hugetlb_cgroup.c
index eec1150..68c2f2f 100644
--- a/mm/hugetlb_cgroup.c
+++ b/mm/hugetlb_cgroup.c
@@ -84,7 +84,7 @@ static void hugetlb_cgroup_init(struct hugetlb_cgroup *h_cgroup,
 
 		limit = round_down(PAGE_COUNTER_MAX,
 				   1 << huge_page_order(&hstates[idx]));
-		ret = page_counter_limit(counter, limit);
+		ret = page_counter_set_max(counter, limit);
 		VM_BUG_ON(ret);
 	}
 }
@@ -273,7 +273,7 @@ static u64 hugetlb_cgroup_read_u64(struct cgroup_subsys_state *css,
 	case RES_USAGE:
 		return (u64)page_counter_read(counter) * PAGE_SIZE;
 	case RES_LIMIT:
-		return (u64)counter->limit * PAGE_SIZE;
+		return (u64)counter->max * PAGE_SIZE;
 	case RES_MAX_USAGE:
 		return (u64)counter->watermark * PAGE_SIZE;
 	case RES_FAILCNT:
@@ -306,7 +306,7 @@ static ssize_t hugetlb_cgroup_write(struct kernfs_open_file *of,
 	switch (MEMFILE_ATTR(of_cft(of)->private)) {
 	case RES_LIMIT:
 		mutex_lock(&hugetlb_limit_mutex);
-		ret = page_counter_limit(&h_cg->hugepage[idx], nr_pages);
+		ret = page_counter_set_max(&h_cg->hugepage[idx], nr_pages);
 		mutex_unlock(&hugetlb_limit_mutex);
 		break;
 	default: