The trace_dump_stack() returned a value for a void function.

Also, added the missing stub for trace_dump_stack() when tracing is
not configured.
Reported-by: NIngo Molnar <mingo@elte.hu>
LKML-Reference: <20091214162713.GA31060@elte.hu>
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>