tracing: Add hist trigger support for stacktraces as keys

It's often useful to be able to use a stacktrace as a hash key, for
keeping a count of the number of times a particular call path resulted
in a trace event, for instance.  Add a special key named 'stacktrace'
which can be used as key in a 'keys=' param for this purpose:

    # echo hist:keys=stacktrace ... \
               [ if filter] > event/trigger

Link: http://lkml.kernel.org/r/87515e90b3785232a874a12156174635a348edb1.1457029949.git.tom.zanussi@linux.intel.com



Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Tested-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>