tracing/function-return-tracer: add a barrier to ensure return stack index is incremented in memory

Impact: fix possible race condition in ftrace function return tracer

This fixes a possible race condition if index incrementation
is not immediately flushed in memory.

Thanks for Andi Kleen and Steven Rostedt for pointing out this issue
and give me this solution.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>