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: Kirill A. Shutemov <[email protected]>
Reviewed-by: Daisuke Nishimura <[email protected]>
Acked-by: KAMEZAWA Hiroyuki <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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);