tracing: Fix regression of trace_options file setting

The latest change to allow trace options to be set on the command
line also broke the trace_options file.

The zeroing of the last byte of the option name that is echoed into
the trace_option file was removed with the consolidation of some
of the code. The compare between the option and what was written to
the trace_options file fails because the string holding the data
written doesn't terminate with a null character.

A zero needs to be added to the end of the string copied from
user space.
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace.c

@@ -2899,6 +2899,8 @@ tracing_trace_options_write(struct file *filp, const char __user *ubuf,
 	if (copy_from_user(&buf, ubuf, cnt))
 		return -EFAULT;
 
+	buf[cnt] = 0;
+
 	trace_set_options(buf);
 
 	*ppos += cnt;