According to the mbuf(9) man page, an inboud packet that contains the
CSUM_DATA_VALID and CSUM_PSEUDO_HDR flags and has it's checksum field set to
0xffff signals that the packet has been validated by the NIC, but that the
actual checksum has not been returned. Translate this flags correctly when
doing packet forwarding, or else the outbound path will wrongly think the
checksum is present.
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