Makes rb_per_cpu_empty() return bool to improve readability.

No functional change.

Link: http://lkml.kernel.org/r/1443537816-5788-6-git-send-email-bywxiaobai@163.comSigned-off-by: NYaowei Bai <bywxiaobai@163.com>
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>