[v3] net/cksum: compute raw cksum for several segments

[Marat Khalili]

> Function rte_raw_cksum_mbuf may produce incorrect result when multiple segments are present in the
> checksummed range, and some of them have odd checksummed length. More precisely, if any segment begins
> at an odd offset from the start, and arithmetic sum of its 16-bit words overflows, the part that has
> overflown will be lost. Cases when there is only one segment, or all of the segment sizes and checksum
> offsets are even, are not affected.

Hi again folks, anything else needed here? There is a test, a fix, and conditions for the issue.
(No problem if it only needs time.)

BTW at least Gemini Pro is able to see this bug (maybe primed by our conversation) and reason about it.