This fixes an endianness bug in `sys/netinet/ip_reass_test`. Just use the code from [[ https://datatracker.ietf.org/doc/html/rfc1071 | RFC 1071. ]].
It might be cleaner to use a function which just computes the checksum and let the caller initialize the `ip_sum` before calling it and store the result in `ip_sum` after the computation has finished.