f2fs: stop issuing discard if fs is readonly

If filesystem is readonly, stop to issue discard in daemon.
Signed-off-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -1406,6 +1406,8 @@ static int issue_discard_thread(void *data)
 				msecs_to_jiffies(wait_ms));
 		if (try_to_freeze())
 			continue;
+		if (f2fs_readonly(sbi->sb))
+			continue;
 		if (kthread_should_stop())
 			return 0;