ring-buffer: rb_is_reader_page() can return boolean

Make rb_is_reader_page() return bool to improve readability due to this
particular function only using either true or false as its return value.

No functional change.

Link: http://lkml.kernel.org/r/1443537816-5788-4-git-send-email-bywxiaobai@163.com



Signed-off-by: Yaowei Bai <bywxiaobai@163.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>