6lowpan: set 6lowpan network and transport header

This is necessary to access network header with the skb_network_header
function instead of calculate the position with mac_len, etc.
Do the same for the transport header, when we replace the IPv6 header
with the 6LoWPAN header.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Acked-by: Werner Almesberger <werner@almesberger.net>
Signed-off-by: David S. Miller <davem@davemloft.net>