net: openthread: filter ipv6 fragments

If OPENTHREAD_IP6_FRAGM is enabled the IPv6 fragments are handled in
OpenThread stack but also forwarder unconditionally to the Zephyr
uplayers. It causes additional packets processing and leads to errors
like unrecognized next header type or duplicate ping reply. What more
these errors generate additional traffic which jam channel and decrease
latency for packets required fragmentation.
This commit add filtering IPv6 fragments when data fragmentation and
reassembling is enabled in OpenThread.

Signed-off-by: Lukasz Maciejonczyk <lukasz.maciejonczyk@nordicsemi.no>