tracing: Set actual size after ring buffer resize

Currently we can resize trace ringbuffer by writing a value into file
'buffer_size_kb', then by reading the file, we get the value that is
usually what we wrote. However, this value may be not actual size of
trace ring buffer because of the round up when doing resize in kernel,
and the actual size would be more useful.

Link: https://lore.kernel.org/linux-trace-kernel/20230705002705.576633-1-zhengyejian1@huawei.com



Cc: <mhiramat@kernel.org>
Signed-off-by: Zheng Yejian <zhengyejian1@huawei.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>