ocfs2/trivial: Use le16_to_cpu for a disk value in xattr.c

In ocfs2_value_metas_in_xattr_header, we should Use
le16_to_cpu for ocfs2_extent_list.l_next_free_rec.

Signed-off-by: Tao Ma <tao.ma@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>