memcg: cleanup root memcg checks

Replace the implicit checking of root memcg with explicit root memcg
checking i.e.  !css->parent with mem_cgroup_is_root().

Link: https://lkml.kernel.org/r/20210223205625.2792891-1-shakeelb@google.com
Signed-off-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Roman Gushchin <guro@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -4123,7 +4123,7 @@ static int mem_cgroup_swappiness_write(struct cgroup_subsys_state *css,
 	if (val > 100)
 		return -EINVAL;
 
-	if (css->parent)
+	if (!mem_cgroup_is_root(memcg))
 		memcg->swappiness = val;
 	else
 		vm_swappiness = val;
@@ -4473,7 +4473,7 @@ static int mem_cgroup_oom_control_write(struct cgroup_subsys_state *css,
 	struct mem_cgroup *memcg = mem_cgroup_from_css(css);
 
 	/* cannot set to root cgroup and only 0 and 1 are allowed */
-	if (!css->parent || !((val == 0) || (val == 1)))
+	if (mem_cgroup_is_root(memcg) || !((val == 0) || (val == 1)))
 		return -EINVAL;
 
 	memcg->oom_kill_disable = val;