memcg: use proper type for mod_memcg_state

memcg: use proper type for mod_memcg_state

The memcg stats update functions can take arbitrary integer but the only
input which make sense is enum memcg_stat_item and we don't want these
functions to be called with arbitrary integer, so replace the parameter
type with enum memcg_stat_item and compiler will be able to warn if memcg
stat update functions are called with incorrect index value.

Link: https://lkml.kernel.org/r/20240501172617.678560-9-shakeel.butt@linux.dev
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Reviewed-by: T.J. Mercier <tjmercier@google.com>
Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>