md/r5cache: call mddev_lock/unlock() in r5c_journal_mode_show

In r5c_journal_mode_show(), it is necessary to call mddev_lock()
before accessing conf and conf->log. Otherwise, the conf->log
may change (and become NULL).
Signed-off-by: Song Liu <songliubraving@fb.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/raid5-cache.c

@@ -2529,11 +2529,18 @@ static void r5l_write_super(struct r5l_log *log, sector_t cp)
 
 static ssize_t r5c_journal_mode_show(struct mddev *mddev, char *page)
 {
-	struct r5conf *conf = mddev->private;
+	struct r5conf *conf;
 	int ret;
 
-	if (!conf->log)
+	ret = mddev_lock(mddev);
+	if (ret)
+		return ret;
+
+	conf = mddev->private;
+	if (!conf || !conf->log) {
+		mddev_unlock(mddev);
 		return 0;
+	}
 
 	switch (conf->log->r5c_journal_mode) {
 	case R5C_JOURNAL_MODE_WRITE_THROUGH:
@@ -2551,6 +2558,7 @@ static ssize_t r5c_journal_mode_show(struct mddev *mddev, char *page)
 	default:
 		ret = 0;
 	}
+	mddev_unlock(mddev);
 	return ret;
 }