net: core: only set l2_processed flag for NET_CONTINUE

If verdict is NET_OK net_pkt may already be unreferenced and reutilized,
so that setting the `l2_processed` flag would be set on a different
packet, corrupting its state. Avoid this situation by only setting the
l2_processed flag for NET_CONTINUE.

Signed-off-by: Tim Pambor <tim.pambor@codewrights.de>