tracing: Simplify trace_option_write()

- remove duplicate code inside trace_options_write()
- extract duplicate code in trace_options_write() and set_tracer_option()

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <4B1DC532.9010802@cn.fujitsu.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>