Simplify the accessPayload() routine so that it always populates the overflow

Simplify the accessPayload() routine so that it always populates the overflow
page cache.  In the one case where populating the page cache can lead to
problems, simply invalidate the cache as soon as accessPayload() returns.
This simplification reduces code size and helps accessPayload() to run a
little faster.  This backs out the eOp==2 mode of accessPayload() added by
check-in [da59198505].

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