There's a problem with bio segment accounting for pages that reside
above the bounce limit of a queue. When submitted, they may be
considered part of another segment. A condition that changes when the
page gets bounced. This can cause us to send bio's that have too many
segments to a driver.

The best fix is to always consider pages above q->bounce_pfn as seperate
segments. That's the conservative approach and the easy fix.

Problem identified and fixed by Herbert Xu.