genet: workaround for problem with ICMPv6 echo replies

The ICMPv6 echo reply is constructed with the IPv6 header too close to
the beginning of a packet for an Ethernet header to be prepended, so we
end up with an mbuf containing just the Ethernet header. The GENET
controller doesn't seem to handle this, with or without transmit checksum
offload. At least until we have chip documentation, do a pullup to
satisfy the chip. Hopefully this can be fixed properly in the future.