net: Change the net_linkaddr struct to not use pointers

Previously the net_linkaddr struct had pointers to the link address.
This is error prone and difficult to handle if cloning the packet as
those pointers can point to wrong place. Mitigate this issue by
allocating the space for link address in net_linkaddr struct. This will
increase the size of the net_pkt by 4 octets for IEEE 802.15.4 where the
link address length is 8, but there no increase in size if link address
is 6 bytes like in Ethernet/Wi-Fi.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>