1. Remove the unused and unimplemented ipv6_chksum declaration
2. Update NET_ARCH_CHKSUM description to align with the implementation
3. Declare all checksum function prototype regardless CONFIG_NET_ARCH_CHKSUM
4. Remove the CONFIG_NET_ARCH_CHKSUM guard for tcp_ipv[4|6]_chksum
net/mld: Checksum calculation needs to know the full size of the IPv6 header, including the size of the extension headers. The payload size in the IPv64 header must include the extension headers (not considered part of the header in this case). Fixes a few additional errors in size, endian-ness and checksum calculations. Wireshark now reports the the outgoing Report has a good checksum.
net/mld/mld_query.c: Add a cast to assure that the left shift does not overflow.
