ftrace: Have ftrace_regex_write() return either read or error

As ftrace_regex_write() reads the result of ftrace_process_regex()
which can sometimes return a positive number, only consider a
failure if the return is negative. Otherwise, it will skip possible
other registered probes and by returning a positive number that
wasn't read, it will confuse the user processes doing the writing.

Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/ftrace.c

@@ -3276,7 +3276,7 @@ ftrace_regex_write(struct file *file, const char __user *ubuf,
 		ret = ftrace_process_regex(iter->hash, parser->buffer,
 					   parser->idx, enable);
 		trace_parser_clear(parser);
-		if (ret)
+		if (ret < 0)
 			goto out_unlock;
 	}