[JFFS2] Put list of nodes in common part of ic/x_ref/x_datum structure

We'll be using a proper list of nodes in the jffs2_xattr_datum and
jffs2_xattr_ref structures, because the existing code to overwrite
them is just broken. Put it in the common part at the front of the
structure which is shared with the jffs2_inode_cache, so that the
jffs2_link_node_ref() function can do the right thing.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>