selftests/bpf: Add csum helpers

Checksum helpers will be used to calculate pseudo-header checksum in
AF_XDP metadata selftests.

The helpers are mirroring existing kernel ones:
- csum_tcpudp_magic : IPv4 pseudo header csum
- csum_ipv6_magic : IPv6 pseudo header csum
- csum_fold : fold csum and do one's complement

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/20231127190319.1190813-11-sdf@google.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>