x86/csum: clean up `csum_partial' further

Commit 688eb819

 ("x86/csum: Improve performance of `csum_partial`")
ended up improving the code generation for the IP csum calculations, and
in particular special-casing the 40-byte case that is a hot case for
IPv6 headers.

It then had _another_ special case for the 64-byte unrolled loop, which
did two chains of 32-byte blocks, which allows modern CPU's to improve
performance by doing the chains in parallel thanks to renaming the carry
flag.

This just unifies the special cases and combines them into just one
single helper the 40-byte csum case, and replaces the 64-byte case by a
80-byte case that just does that single helper twice.  It avoids having
all these different versions of inline assembly, and actually improved
performance further in my tests.

There was never anything magical about the 64-byte unrolled case, even
though it happens to be a common size (and typically is the cacheline
size).

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>