[NETROM]: Fix rebuild header mess

For reason that probably nobody recalls NET/ROM does it's actual
packet transmission in nr_rebuild_header and even treats invocation of
it's hard_start_xmit method nr_xmit as a bug.  Fix that by splitting
the job done by nr_rebuild_header into two halves.  Along with that we
now also can get rid of the silly clone of the skb on transmit.

Signed-off-by: Ralf Baechle DL5RB <ralf@linux-mips.org>
Signed-off-by: David S. Miller <davem@davemloft.net>