perf ftrace: Flush output after each writing

The pager will result stdout in full buffering mode instead of line
buffering. We need to make the trace visible timely.
Signed-off-by: Changbin Du <changbin.du@huawei.com>
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: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230513074000.733550-1-changbin.du@huawei.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-ftrace.c

@@ -650,6 +650,8 @@ static int __cmd_ftrace(struct perf_ftrace *ftrace)
 				break;
 			if (fwrite(buf, n, 1, stdout) != 1)
 				break;
+			/* flush output since stdout is in full buffering mode due to pager */
+			fflush(stdout);
 		}
 	}