tracing: cleanup for tracing_trace_options_read()

'\n' is already appended, and what we need is just an extra
space for the '\0'.

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
LKML-Reference: <4A3EED63.3090908@cn.fujitsu.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>