chacha20: Reorder MAC verification and decryption process

[matbech]

Description

Refactor decryption logic to verify MAC before decrypting data. In addition, it removes the unused allzero variable and chacha20_ismemset check.

Problem Statement

• verify MAC before decrypting data
• remove unused allzero variable and check

Type of Change

Please select the type of change:

• Refactor

Checklist

• I have independently verified the issue
• I am not submitting unverified or speculative changes
• I understand that AI-assisted changes must be reviewed by a human before submission

[utelle]

Refactor decryption logic to verify MAC before decrypting data.

For normal operation the order doesn't matter, but in the event of an error it makes sense. Most likely the same logic should be applied to other cipher schemes with MACs.

In addition, it removes the unused allzero variable and chacha20_ismemset check.

Most likely I was distracted somehow back then when I added this test. AFAICR a special case can occur where the page is allzero, and maybe that should be handled explicitly. I have to dig in my notes to be sure.