f2fs: do not make dirty any inmemory pages

This patch let inmemory pages be clean all the time.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index 8171e80..28f24ea 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -269,6 +269,7 @@
 	struct extent_info ext;		/* in-memory extent cache entry */
 	struct dir_inode_entry *dirty_dir;	/* the pointer of dirty dir */
 
+	struct radix_tree_root inmem_root;	/* radix tree for inmem pages */
 	struct list_head inmem_pages;	/* inmemory pages managed by f2fs */
 	struct mutex inmem_lock;	/* lock for inmemory pages */
 };