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.comSigned-off-by: NWan Jiabing <wanjiabing@vivo.com>
Reviewed-by: NMuchun Song <songmuchun@bytedance.com>
Acked-by: NMichal Hocko <mhocko@suse.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>