memcg: reduce memory for the lruvec and memcg stats

At the moment, the amount of memory allocated for stats related structs in
the mem_cgroup corresponds to the size of enum node_stat_item.  However
not all fields in enum node_stat_item have corresponding memcg stats.  So,
let's use indirection mechanism similar to the one used for memcg vmstats
management.

For a given x86_64 config, the size of stats with and without patch is:

structs size in bytes         w/o     with

struct lruvec_stats           1128     648
struct lruvec_stats_percpu     752     432
struct memcg_vmstats          1832    1352
struct memcg_vmstats_percpu   1280     960

The memory savings are further compounded by the fact that these structs
are allocated for each cpu and for each node.  To be precise, for each
memcg the memory saved would be:

Memory saved = ((21 * 3 * NR_NODES) + (21 * 2 * NR_NODES * NR_CPUS) +
	       (21 * 3) + (21 * 2 * NR_CPUS)) * sizeof(long)

Where 21 is the number of fields eliminated.

Link: https://lkml.kernel.org/r/20240501172617.678560-5-shakeel.butt@linux.dev


Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
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: T.J. Mercier <tjmercier@google.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>