6lowpan: use skb_copy_linear_data instead offset

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
6lowpan: add timer functionalty cleanup some code

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
6lowpan: fix udp compression function

Signed-off-by: Alexander Aring <alex.aring@gmail.com>