tracing: A minor cleanup for create_system_filter()

The first two parameters should be reduced to one, as @tr is simply
@dir->tr.

Link: https://lkml.kernel.org/r/20210324205642.65e03248@oasis.local.home
Link: https://lkml.kernel.org/r/20210325163752.128407-1-hqjagain@gmail.com



Suggested-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Qiujun Huang <hqjagain@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>