Deprecate loader tunable fixup_needs_csum of the vtnet interface.
If this tunable is enabled and vtnet receives a packet with VIRTIO_NET_HDR_F_NEEDS_CSUM set, vtnet computes the TCP/UDP checksum and writes it in the checksum field. This was somewhat useful in the time when vtnet pretended that such a packet has a correct checksum and set the mbuf flag CSUM_DATA_VALID. However, since D51686, this is not the case anymore.
Furthermore, the guest can avoid receiving packets with an incorrect checksum and VIRTIO_NET_HDR_F_NEEDS_CSUM set by disabling the feature VIRTIO_NET_F_GUEST_CSUM. With vtnet, this is possible by disabling RXCSUM on the interface.