ieee802154: handle datagram variables as u16

This reverts commit 9abc378c66e3d6f437eed77c1c534cbc183523f7
("ieee802154: 6lowpan: change datagram var types").

The reason is that I forgot the IPv6 fragmentation here. Our MTU of
lowpan interface is 1280 and skb->len should not above of that. If we
reach a payload above 1280 in IPv6 header then we have a IPv6
fragmentation above 802.15.4 6LoWPAN fragmentation. The type "u16" was
fine, instead I added now a WARN_ON_ONCE if skb->len is above MTU which
should never happen otherwise IPv6 on minimum MTU size is broken.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>