f2fs: avoid freqeunt write_inode calls

If update_inode is called, we don't need to do write_inode.
So, let's use a *dirty* flag for each inode.

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index 4fdcdff..ba56549 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -112,6 +112,17 @@
 	return generic_drop_inode(inode);
 }
 
+/*
+ * f2fs_dirty_inode() is called from __mark_inode_dirty()
+ *
+ * We should call set_dirty_inode to write the dirty inode through write_inode.
+ */
+static void f2fs_dirty_inode(struct inode *inode, int flags)
+{
+	set_inode_flag(F2FS_I(inode), FI_DIRTY_INODE);
+	return;
+}
+
 static void f2fs_i_callback(struct rcu_head *head)
 {
 	struct inode *inode = container_of(head, struct inode, i_rcu);
@@ -249,6 +260,7 @@
 	.drop_inode	= f2fs_drop_inode,
 	.destroy_inode	= f2fs_destroy_inode,
 	.write_inode	= f2fs_write_inode,
+	.dirty_inode	= f2fs_dirty_inode,
 	.show_options	= f2fs_show_options,
 	.evict_inode	= f2fs_evict_inode,
 	.put_super	= f2fs_put_super,