Make ring_buffer_empty() and ring_buffer_empty_cpu() return bool.

No functional change.

Link: http://lkml.kernel.org/r/1443537816-5788-5-git-send-email-bywxiaobai@163.comSigned-off-by: NYaowei Bai <bywxiaobai@163.com>
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>