struct mem_cgroup is declared twice.  One has been declared at forward
struct declaration.  Remove the duplicate.
Link: https://lkml.kernel.org/r/20210330020246.2265371-1-wanjiabing@vivo.com
Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
 #define MEM_CGROUP_ID_SHIFT    0
 #define MEM_CGROUP_ID_MAX      0
 
-struct mem_cgroup;
-
 static inline struct mem_cgroup *page_memcg(struct page *page)
 {
        return NULL;