6lowpan: cleanup skb copy data

This patch drops the direct memcpy on skb and uses the right skb
memcpy functions. Also remove an unnecessary check if plen is non zero.

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