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/file.c b/fs/f2fs/file.c
index 2f649b8..fda226f 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -147,6 +147,7 @@
 	} else {
 		/* if there is no written node page, write its inode page */
 		while (!sync_node_pages(sbi, inode->i_ino, &wbc)) {
+			mark_inode_dirty_sync(inode);
 			ret = f2fs_write_inode(inode, NULL);
 			if (ret)
 				goto out;