f2fs: add f2fs_readonly()

Introduce a simple macro function for readability.

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index 8555f7d..3ac305d 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -170,7 +170,7 @@
 {
 	int err;
 
-	if (sb->s_flags & MS_RDONLY)
+	if (f2fs_readonly(sb))
 		return 0;
 
 	err = f2fs_sync_fs(sb, 1);