MFC r260702 (by melifaro):

MFC r260702 (by melifaro):
  Fix ipfw fwd for IPv4 traffic broken by r249894.

  Problem case:
  Original lookup returns route with GW set, so gw points to
  rte->rt_gateway.
  After that we're changing dst and performing lookup another time.
  Since fwd host is most probably directly reachable, resulting
  rte does not contain rt_gateway, so gw is not set. Finally, we
  end with packet transmitted to proper interface but wrong
  link-layer address.

git-svn-id: svn://svn.freebsd.org/base/stable/10@261545 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f