net: ip: Speed up the IP/UDP/TCP checksum calculation

Instead of doing a 1-complement addition for every 16-bit word, process
32-bit words and handle the 1-complement addition at the end.

This work is a colaboration with Diego Pino García. The algorithm is
inspired by:
https://blogs.igalia.com/dpino/2018/06/14/fast-checksum-computation/



Signed-off-by: Sjors Hettinga <s.a.hettinga@gmail.com>