Use pr_warn_once, instead of making an open coded implementation.

Link: http://lkml.kernel.org/r/513D8419.20400@huawei.comSigned-off-by: Nzhangwei(Jovi) <jovi.zhangwei@huawei.com>
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>