f2fs: stop issuing discard if fs is readonly

If filesystem is readonly, stop to issue discard in daemon.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index 35ab28b..9fa5d70 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -1406,6 +1406,8 @@
 				msecs_to_jiffies(wait_ms));
 		if (try_to_freeze())
 			continue;
+		if (f2fs_readonly(sbi->sb))
+			continue;
 		if (kthread_should_stop())
 			return 0;