mm: memcg: put struct task_struct::memcg_in_oom under CONFIG_MEMCG_V1

mm: memcg: put struct task_struct::memcg_in_oom under CONFIG_MEMCG_V1

The memcg_in_oom field of the struct task_struct is not used by the cgroup
v2's memory controller, so it can be happily compiled out if
CONFIG_MEMCG_V1 is not set.

Link: https://lkml.kernel.org/r/20240628210317.272856-9-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>