cam/da: explain why we have to refetch bp here.

This looks redundant. However, daerror calls cmd6workaround which will
sets ccb_bp = NULL when we're failing over to a new delete method, since
different delete methods have different size limitations. Unlike other
error recovery, we can't just change the CDB and retry because of the
latter, so we requeue it. Add a coment explaining this, lest anybody
(else) be tempted to remove it.

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