tracing: Fix off-by-one in trace_get_user()

Leave the last slot for the tailing '\0'.
Signed-off-by: NLi Zefan <lizf@cn.fujitsu.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <4AB865FA.5080801@cn.fujitsu.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/trace/trace.c

@@ -415,7 +415,7 @@ int trace_get_user(struct trace_parser *parser, const char __user *ubuf,
 
 	/* read the non-space input */
 	while (cnt && !isspace(ch)) {
-		if (parser->idx < parser->size)
+		if (parser->idx < parser->size - 1)
 			parser->buffer[parser->idx++] = ch;
 		else {
 			ret = -EINVAL;