tracing: Add helper function tracing_is_disabled()

This patch creates the function 'tracing_is_disabled', which
can be used outside of trace.c.

Link: http://lkml.kernel.org/r/1382141754-12155-1-git-send-email-geyslan@gmail.com



Signed-off-by: Geyslan G. Bem <geyslan@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>