relayfs: support a counter tracking if data is too big to write

relayfs: support a counter tracking if data is too big to write

It really doesn't matter if the user/admin knows what the last too big
value is.  Record how many times this case is triggered would be helpful.

Solve the existing issue where relay_reset() doesn't restore the value.

Store the counter in the per-cpu buffer structure instead of the global
buffer structure.  It also solves the racy condition which is likely to
happen when a few of per-cpu buffers encounter the too big data case and
then access the global field last_toobig without lock protection.

Remove the printk in relay_close() since kernel module can directly call
relay_stats() as they want.

Link: https://lkml.kernel.org/r/20250612061201.34272-6-kerneljasonxing@gmail.com
Signed-off-by: Jason Xing <kernelxing@tencent.com>
Reviewed-by: Yushan Zhou <katrinzhou@tencent.com>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>