mm: swap: use mem_cgroup_is_root() instead of deferencing css->parent

mem_cgroup_is_root() is the preferred API to check if memcg is root or
not.  Use it instead of deferencing css->parent.

Link: http://lkml.kernel.org/r/1547232913-118148-1-git-send-email-yang.shi@linux.alibaba.comSigned-off-by: Yang Shi <yang.shi@linux.alibaba.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Tim Chen <tim.c.chen@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/swap.h

@@ -625,7 +625,7 @@ static inline int mem_cgroup_swappiness(struct mem_cgroup *memcg)
 		return vm_swappiness;
 
 	/* root ? */
-	if (mem_cgroup_disabled() || !memcg->css.parent)
+	if (mem_cgroup_disabled() || mem_cgroup_is_root(memcg))
 		return vm_swappiness;
 
 	return memcg->swappiness;