6lowpan: set and use mac_len for mac header length

Set the mac header length while creation of the 802.15.4 mac header.
Drop the function for recalculation mac_len in upper layers.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
6lowpan: clean lowpan setup

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
6lowpan: remove several copies of sk_buff

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
6lowpan: fix fragmentation on sending side

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
6lowpan: allow to skip bytes

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

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

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