tracing: Prevent buffer overwrite disabled for latency tracers

tracing: Prevent buffer overwrite disabled for latency tracers

commit 613f04a0f51e6e68ac6fe571ab79da3c0a5eb4da upstream.

The latency tracers require the buffers to be in overwrite mode,
otherwise they get screwed up. Force the buffers to stay in overwrite
mode when latency tracers are enabled.

Added a flag_changed() method to the tracer structure to allow
the tracers to see what flags are being changed, and also be able
to prevent the change from happing.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
[bwh: Backported to 3.2:
 - Adjust context
 - Drop some changes that are not needed because trace_set_options() is not
   separate from tracing_trace_options_write()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>