net: ip: Set net_context on the final fragment

IPv4/6 fragmentation did not set the net_context pointer on the fragment
packet and in result the send callback registered on net_context was not
called. Therefore, copy the net_context pointer from the original packet
to the final fragment to ensure that the registered callback is called.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>