Ipv6 payload length doesn't include header length.
rfc8200 section 3
https://tools.ietf.org/html/rfc8200
Payload Length 16-bit unsigned integer. Length of the IPv6
payload, i.e., the rest of the packet
following this IPv6 header, in octets.
Bug: 215655463
Test: atest NetworkStaticLibTests
Change-Id: Ifdc871125b0fe2eb3a8909b8b0723a62dbe44bcf