Fix issue with dst bias in memset

Fix issue with dst bias in memset

This patch fixes an issue in the previous memset loop change. If the
zva size is >= 256 and there are more than 64 bytes left in the
tail, we could enter the loop and thus need to rebias dst by 32 as
well.

Since no known CPUs use this size this can't be tested natively, so I've
tested it on a simulator initialized with a large zva size.

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