perf ftrace: Factor out function write_tracing_file_int()

We will reuse this function later.

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
Link: http://lore.kernel.org/lkml/20200808023141.14227-4-changbin.du@gmail.com


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