6lowpan: Make a copy of skb's delivered to 6lowpan

Since lowpan_process_data() modifies the skb (by calling skb_pull()), we
need our own copy so that it doesn't affect the data received by other
protcols (in this case, af_ieee802154).

Signed-off-by: Alan Ott <alan@signal11.us>
Signed-off-by: David S. Miller <davem@tempietto.lan>