ring-buffer: allow tracing_off to be used in core kernel code

tracing_off() is the fastest way to stop recording to the ring buffers.
This may be used in places like panic and die, just before the
ftrace_dump is called.

This patch adds the appropriate CPP conditionals to make it a stub
function when the ring buffer is not configured it.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>