tracing: Use pr_warn_once instead of open coded implementation

Use pr_warn_once, instead of making an open coded implementation.

Link: http://lkml.kernel.org/r/513D8419.20400@huawei.com



Signed-off-by: zhangwei(Jovi) <jovi.zhangwei@huawei.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>