memcg: fix memory.memsw.usage_in_bytes for root cgroup

A memory cgroup has a memory.memsw.usage_in_bytes file.  It shows the sum
of the usage of pages and swapents in the cgroup.  Presently the root
cgroup's memsw.usage_in_bytes shows the wrong value - the number of
swapents are not added.

So take MEM_CGROUP_STAT_SWAPOUT into account.
Signed-off-by: NKirill A. Shutemov <kirill@shutemov.name>
Reviewed-by: NDaisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Acked-by: NKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: <stable@kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -2542,6 +2542,7 @@ static u64 mem_cgroup_read(struct cgroup *cont, struct cftype *cft)
 			val += idx_val;
 			mem_cgroup_get_recursive_idx_stat(mem,
 				MEM_CGROUP_STAT_SWAPOUT, &idx_val);
+			val += idx_val;
 			val <<= PAGE_SHIFT;
 		} else
 			val = res_counter_read_u64(&mem->memsw, name);