net: ipv6: fragmentation: Drop the packet if fragmentation fails

If we could not fragment the IPv6 packet, then drop it and do
not try to send it. Let the upper layer re-send the packet if
needed. It is causing more trouble if we try to send the packet
and not honor the MTU setting.

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