tracing: Add a trace print when traceoff_on_warning is triggered

When "traceoff_on_warning" is enabled and a warning happens, there can still
be many trace events happening on other CPUs between the time the warning
occurred and the last trace event on that same CPU. This can cause confusion
in examining the trace, as it may not be obvious where the warning happened.
By adding a trace print into the trace just before disabling tracing, it
makes it obvious where the warning occurred, and the developer doesn't have
to look at other means to see what CPU it occurred on.

Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>