crypto: aesni - fix counter overflow handling in "by8" variant

The "by8" CTR AVX implementation fails to propperly handle counter
overflows. That was the reason it got disabled in commit 7da4b29d496b
("crypto: aesni - disable "by8" AVX CTR optimization").

Fix the overflow handling by incrementing the counter block as a double
quad word, i.e. a 128 bit, and testing for overflows afterwards. We need
to use VPTEST to do so as VPADD* does not set the flags itself and
silently drops the carry bit.

As this change adds branches to the hot path, minor performance
regressions  might be a side effect. But, OTOH, we now have a conforming
implementation -- the preferable goal.

A tcrypt test on a SandyBridge system (i7-2620M) showed almost identical
numbers for the old and this version with differences within the noise
range. A dm-crypt test with the fixed version gave even slightly better
results for this version. So the performance impact might not be as big
as expected.

Tested-by: Romain Francoise <[email protected]>
Signed-off-by: Mathias Krause <[email protected]>
Cc: Chandramouli Narayanan <[email protected]>
Signed-off-by: Herbert Xu <[email protected]>

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