relayfs: support a counter tracking if per-cpu buffers is full

When using relay mechanism, we often encounter the case where new data are
lost or old unconsumed data are overwritten because of slow reader.

Add 'full' field in per-cpu buffer structure to detect if the above case
is happening.  Relay has two modes: 1) non-overwrite mode, 2) overwrite
mode.  So buffer being full here respectively means: 1) relayfs doesn't
intend to accept new data and then simply drop them, or 2) relayfs is
going to start over again and overwrite old unread data with new data.

Note: this counter doesn't need any explicit lock to protect from being
modified by different threads for the better performance consideration. 
Writers calling __relay_write/relay_write should consider how to use the
lock and ensure it performs under the lock protection, thus it's not
necessary to add a new small lock here.

Link: https://lkml.kernel.org/r/20250612061201.34272-3-kerneljasonxing@gmail.com


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