ring-buffer: use READ_ONCE() to read cpu_buffer->commit_page in concurrent environment

ring-buffer: use READ_ONCE() to read cpu_buffer->commit_page in concurrent environment

In function ring_buffer_iter_empty(), cpu_buffer->commit_page is read
while other threads may change it. It may cause the time_stamp that read
in the next line come from a different page. Use READ_ONCE() to avoid
having to reason about compiler optimizations now and in future.

Link: https://lore.kernel.org/linux-trace-kernel/[email protected]
Cc: Masami Hiramatsu <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Signed-off-by: linke li <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>