mm: memcg: adjust the warning when seq_buf overflows

Currently it uses WARN_ON_ONCE() if seq_buf overflows when user reads
memory.stat, the only advantage of WARN_ON_ONCE is that the splat is so
verbose that it gets noticed. And also it panics the system if
panic_on_warn is enabled. It seems like the warning is just an over
reaction and a simple pr_warn should just achieve the similar effect.

Link: https://lkml.kernel.org/r/20240628072333.2496527-1-xiujianfeng@huawei.com
Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Suggested-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>