UPSTREAM: tracefs: Add directory /sys/kernel/tracing

When tracefs is configured, have the directory /sys/kernel/tracing appear
just like /sys/kernel/debug appears when debugfs is configured.

This will give a consistent place for system admins to mount tracefs.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

Bug: 31856701
Change-Id: I18ccb9c55d2397a36c8eaa0d2621f784335a3df2
(cherry picked from commit cc31004a4aa784d89054ec07b87eae05cecf7121)
Signed-off-by: Paul Lawrence <paullawrence@google.com>