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/file.c b/fs/f2fs/file.c
index 9937ba1..316bcfe 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -114,7 +114,7 @@
 		.for_reclaim = 0,
 	};
 
-	if (inode->i_sb->s_flags & MS_RDONLY)
+	if (f2fs_readonly(inode->i_sb))
 		return 0;
 
 	trace_f2fs_sync_file_enter(inode);