tracing: is_legal_op() can return boolean

Make is_legal_op() return bool to improve readability due to this particular
function only using either one or zero as its return value.

No functional change.

Link: http://lkml.kernel.org/r/1443537816-5788-8-git-send-email-bywxiaobai@163.com



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