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.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>