blktrace: use rbuf->stats.full as a drop indicator in relayfs

Replace internal subbuf_start in blktrace with the default policy in
relayfs.

Remove dropped field from struct blktrace.  Correspondingly, call the
common helper in relay.  By incrementing full_count to keep track of how
many times we encountered a full buffer issue, user space will know how
many events were lost.

Link: https://lkml.kernel.org/r/20250612061201.34272-5-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>