f2fs: fix inconsistency between xattr node blocks and its inode

Previously xattr node blocks are stored to the COLD_NODE log, which means that
our roll-forward mechanism doesn't recover the xattr node blocks at all.
Only the direct node blocks in the WARM_NODE log can be recovered.

So, let's resolve the issue simply by conducting checkpoint during fsync when a
file has a modified xattr node block.

This approach is able to degrade the performance, but normally the checkpoint
overhead is shown at the initial fsync call after the xattr entry changes.
Once the checkpoint is done, no additional overhead would be occurred.

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/f2fs.h

@@ -181,6 +181,7 @@ struct f2fs_inode_info {
 	f2fs_hash_t chash;		/* hash value of given file name */
 	unsigned int clevel;		/* maximum level of given file name */
 	nid_t i_xattr_nid;		/* node id that contains xattrs */
+	unsigned long long xattr_ver;	/* cp version of xattr modification */
 	struct extent_info ext;		/* in-memory extent cache entry */
 };