mm/memcontrol: remove redundant NULL check

Fix below warnings reported by coccicheck:

  mm/memcontrol.c:451:3-9: WARNING: NULL check before some freeing functions is not needed.

Link: https://lkml.kernel.org/r/1611216029-34397-1-git-send-email-abaci-bugfix@linux.alibaba.comSigned-off-by: Yang Li <abaci-bugfix@linux.alibaba.com>
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -452,8 +452,7 @@ static void memcg_free_shrinker_maps(struct mem_cgroup *memcg)
 	for_each_node(nid) {
 		pn = mem_cgroup_nodeinfo(memcg, nid);
 		map = rcu_dereference_protected(pn->shrinker_map, true);
-		if (map)
-			kvfree(map);
+		kvfree(map);
 		rcu_assign_pointer(pn->shrinker_map, NULL);
 	}
 }