memcg: writeback: use memcg->cgwb_list directly

mem_cgroup_cgwb_list is a very simple wrapper and it will never be used
outside of code under CONFIG_CGROUP_WRITEBACK.  so use memcg->cgwb_list
directly.

Link: http://lkml.kernel.org/r/1524406173-212182-1-git-send-email-wanglong19@meituan.com
Signed-off-by: Wang Long <wanglong19@meituan.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/memcontrol.h

@@ -1097,7 +1097,6 @@ static inline void dec_lruvec_page_state(struct page *page,
 
 #ifdef CONFIG_CGROUP_WRITEBACK
 
-struct list_head *mem_cgroup_cgwb_list(struct mem_cgroup *memcg);
 struct wb_domain *mem_cgroup_wb_domain(struct bdi_writeback *wb);
 void mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
 			 unsigned long *pheadroom, unsigned long *pdirty,

mm/backing-dev.c

@@ -557,7 +557,7 @@ static int cgwb_create(struct backing_dev_info *bdi,
 	memcg = mem_cgroup_from_css(memcg_css);
 	blkcg_css = cgroup_get_e_css(memcg_css->cgroup, &io_cgrp_subsys);
 	blkcg = css_to_blkcg(blkcg_css);
-	memcg_cgwb_list = mem_cgroup_cgwb_list(memcg);
+	memcg_cgwb_list = &memcg->cgwb_list;
 	blkcg_cgwb_list = &blkcg->cgwb_list;
 
 	/* look up again under lock and discard on blkcg mismatch */
@@ -736,7 +736,7 @@ static void cgwb_bdi_unregister(struct backing_dev_info *bdi)
  */
 void wb_memcg_offline(struct mem_cgroup *memcg)
 {
-	struct list_head *memcg_cgwb_list = mem_cgroup_cgwb_list(memcg);
+	struct list_head *memcg_cgwb_list = &memcg->cgwb_list;
 	struct bdi_writeback *wb, *next;
 
 	spin_lock_irq(&cgwb_lock);

mm/memcontrol.c

@@ -3562,11 +3562,6 @@ static int mem_cgroup_oom_control_write(struct cgroup_subsys_state *css,
 
 #ifdef CONFIG_CGROUP_WRITEBACK
 
-struct list_head *mem_cgroup_cgwb_list(struct mem_cgroup *memcg)
-{
-	return &memcg->cgwb_list;
-}
-
 static int memcg_wb_domain_init(struct mem_cgroup *memcg, gfp_t gfp)
 {
 	return wb_domain_init(&memcg->cgwb_domain, gfp);