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

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

The struct task_struct's in_user_fault member is not used by the cgroup
v2's memory controller, so it can be put under the CONFIG_MEMCG_V1 config
option.  To do so, mem_cgroup_enter_user_fault() and
mem_cgroup_exit_user_fault() are moved under the CONFIG_MEMCG_V1 option as
well.

Link: https://lkml.kernel.org/r/20240628210317.272856-10-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>