perf ftrace: Make system wide the default target for latency subcommand

If no target is specified for 'latency' subcommand, the execution fails
because - 1 (invalid value) is written to set_ftrace_pid tracefs file.
Make system wide the default target, which is the same as the default
behavior of 'trace' subcommand.

Before the fix:

  # perf ftrace latency -T schedule
  failed to set ftrace pid

After the fix:

  # perf ftrace latency -T schedule
  ^C#   DURATION     |      COUNT | GRAPH                                          |
       0 - 1    us |          0 |                                                |
       1 - 2    us |          0 |                                                |
       2 - 4    us |          0 |                                                |
       4 - 8    us |       2828 | ####                                           |
       8 - 16   us |      23953 | ########################################       |
      16 - 32   us |        408 |                                                |
      32 - 64   us |        318 |                                                |
      64 - 128  us |          4 |                                                |
     128 - 256  us |          3 |                                                |
     256 - 512  us |          0 |                                                |
     512 - 1024 us |          1 |                                                |
       1 - 2    ms |          4 |                                                |
       2 - 4    ms |          0 |                                                |
       4 - 8    ms |          0 |                                                |
       8 - 16   ms |          0 |                                                |
      16 - 32   ms |          0 |                                                |
      32 - 64   ms |          0 |                                                |
      64 - 128  ms |          0 |                                                |
     128 - 256  ms |          4 |                                                |
     256 - 512  ms |          2 |                                                |
     512 - 1024 ms |          0 |                                                |
       1 - ...   s |          0 |                                                |

Fixes: 53be5028 ("perf ftrace: Add 'latency' subcommand")
Signed-off-by: Yang Jihong <yangjihong1@huawei.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230324032702.109964-1-yangjihong1@huawei.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-ftrace.c

@@ -1228,10 +1228,12 @@ int cmd_ftrace(int argc, const char **argv)
 		goto out_delete_filters;
 	}
 
-	switch (subcmd) {
-	case PERF_FTRACE_TRACE:
+	/* Make system wide (-a) the default target. */
 	if (!argc && target__none(&ftrace.target))
 		ftrace.target.system_wide = true;
+
+	switch (subcmd) {
+	case PERF_FTRACE_TRACE:
 		cmd_func = __cmd_ftrace;
 		break;
 	case PERF_FTRACE_LATENCY: