tracing: Fix traceoff_on_warning handling on boot command line

According to the documentation, adding "traceoff_on_warning" to the boot
command line should be enough to enable the feature. But right now it is
necessary to specify "traceoff_on_warning=". Along with fixing that, also
verify if the value passed, if any, is either "0" or "off".

Link: http://lkml.kernel.org/r/20141112231400.GL12281@uudg.org
Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>