Remove the ipre_mlast field and the TRAVERSE macro.

The goal was to store in ipre_mlast the last mbuf of the chain, so that
m_cat could be called on it. But it's not needed, since m_cat already
does the equivalent of TRAVERSE itself.

If it were needed, there would be a bug, since we don't call TRAVERSE on
ipre_mlast when creating a new reassembly entry.

(maxv)