RE: x86/csum: Remove unnecessary odd handling

[David Laight]

...
> > All csum kunit tests pass.

Last time I looked I couldn't see where generated IPv6
checksums get changed from 0x0000 (from ~csum_fold() using
adc) to 0xffff - which I think the protocol requires.

The trivial way to do this is to initialise the sum to 1
(instead or 0 or 0xffff) and then add 1 after the invert.

It doesn't matter (much) for IPv4 because 0x0000 is 'no checksum'
rather than 'invalid'.

	David

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