mm: memcg: gather memcg1-specific fields initialization in memcg1_memcg_init()

Gather all memcg1-specific struct mem_cgroup's members initialization in a
new memcg1_memcg_init() function, defined in mm/memcontrol-v1.c. 
Obviously, if CONFIG_MEMCG_V1 is not set, there is no need to initialize
these fields, so the function becomes trivial.

Link: https://lkml.kernel.org/r/20240628210317.272856-5-roman.gushchin@linux.dev


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