memcg: move do_memsw_account() to CONFIG_MEMCG_V1

memcg: move do_memsw_account() to CONFIG_MEMCG_V1

The do_memsw_account() is used to enable or disable legacy memory+swap
accounting in memory cgroup.  However with disabled CONFIG_MEMCG_V1, we
don't need to keep checking it.  So, let's always return false for
!CONFIG_MEMCG_V1 configs.

Before the patch:

$ size mm/memcontrol.o
   text    data     bss     dec     hex filename
  49928   10736    4172   64836    fd44 mm/memcontrol.o

After the patch:

$ size mm/memcontrol.o
   text    data     bss     dec     hex filename
  49430   10480    4172   64082    fa52 mm/memcontrol.o

Link: https://lkml.kernel.org/r/20250312222552.3284173-1-shakeel.butt@linux.dev
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>