perf ftrace: Make 'function_graph' be the default tracer

So that we can suppress the '-t function_graph' and get a more compact command
line:

  # perf ftrace usleep 123456 | grep raw_spin_lock | sort -k2 -nr | head -5
  2)   0.555 us    |                _raw_spin_lock();
  2)   0.516 us    |          _raw_spin_lock();
  2)   0.410 us    |          _raw_spin_lock_irq();
  2)   0.374 us    |                        _raw_spin_lock_irqsave();
  #
Tested-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jeremy Eder <jeder@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-ss9xgx5htpxcv86x42pnh3m6@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-ftrace.c

@@ -202,6 +202,7 @@ int cmd_ftrace(int argc, const char **argv, const char *prefix __maybe_unused)
 {
 	int ret;
 	struct perf_ftrace ftrace = {
+		.tracer = "function_graph",
 		.target = { .uid = UINT_MAX, },
 	};
 	const char * const ftrace_usage[] = {
@@ -211,7 +212,7 @@ int cmd_ftrace(int argc, const char **argv, const char *prefix __maybe_unused)
 	};
 	const struct option ftrace_options[] = {
 	OPT_STRING('t', "tracer", &ftrace.tracer, "tracer",
-		   "tracer to use: function_graph or function"),
+		   "tracer to use: function_graph(default) or function"),
 	OPT_INCR('v', "verbose", &verbose,
 		 "be more verbose"),
 	OPT_END()