memcg: fix typo in kmemcg cache walk macro

The macro for_each_memcg_cache_index contains a silly yet potentially
deadly mistake. Although the macro parameter is _idx, the loop tests
are done over i, not _idx.

This hasn't generated any problems so far, because all users use i as a
loop index. However, while playing with an extension of the code I
ended using another loop index and the compiler was quick to complain.

Unfortunately, this is not the kind of thing that testing reveals =(

Signed-off-by: Glauber Costa <[e-mail]>
Cc: Kamezawa Hiroyuki <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/memcontrol.h

···
  * the slab_mutex must be held when looping through those caches
  */
 #define for_each_memcg_cache_index(_idx)	\
-	for ((_idx) = 0; i < memcg_limited_groups_array_size; (_idx)++)
+	for ((_idx) = 0; (_idx) < memcg_limited_groups_array_size; (_idx)++)
 
 static inline bool memcg_kmem_enabled(void)
 {