x86/csum: Rewrite/optimize csum_partial()

With more NICs supporting CHECKSUM_COMPLETE, and IPv6 being widely
used csum_partial() is heavily used with small amount of bytes, and is
consuming many cycles.

IPv6 header size, for instance, is 40 bytes.

Another thing to consider is that NET_IP_ALIGN is 0 on x86, meaning
that network headers are not word-aligned, unless the driver forces
this.

This means that csum_partial() fetches one u16 to 'align the buffer',
then performs three u64 additions with carry in a loop, then a
remaining u32, then a remaining u16.

With this new version, it performs a loop only for the 64 bytes blocks,
then the remaining is bisected.

Testing on various CPUs, all of them show a big reduction in
csum_partial() cost (by 50 to 80 %)

Before:
	4.16%  [kernel]       [k] csum_partial
After:
	0.83%  [kernel]       [k] csum_partial

If run in a loop 1,000,000 times:

Before:
	26,922,913      cycles                    # 3846130.429 GHz
	80,302,961      instructions              #    2.98  insn per cycle
	21,059,816      branches                  # 3008545142.857 M/sec
	     2,896      branch-misses             #    0.01% of all branches
After:
	17,960,709      cycles                    # 3592141.800 GHz
	41,292,805      instructions              #    2.30  insn per cycle
	11,058,119      branches                  # 2211623800.000 M/sec
	     2,997      branch-misses             #    0.03% of all branches

 [ bp: Massage, merge in subsequent fixes into a single patch:
   - um compilation error due to missing load_unaligned_zeropad():
- Reported-by: kernel test robot <lkp@intel.com>
	- Link: https://lkml.kernel.org/r/20211118175239.1525650-1-eric.dumazet@gmail.com


   - Fix initial seed for odd buffers
- Reported-by: Noah Goldstein <goldstein.w.n@gmail.com>
	- Link: https://lkml.kernel.org/r/20211125141817.3541501-1-eric.dumazet@gmail.com


  ]

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Link: https://lore.kernel.org/r/20211112161950.528886-1-eric.dumazet@gmail.com