powerpc/dma: Fix accounting and loading large segments

[jhibbits]

Summary

When the first page of a segment fits alignment, the second likely does
not, so the DMA infrastructure (must_bounce()) thinks it needs to bounce
the pages. Fix this by passing the previous end (address of byte
following the previous segment) as a third argument to must_bounce(), so
that the alignment check is done against the start of a new segment if
and only if necessary, instead of the current page.

[jhibbits]

Helps if I add some reviewers.

[adrian]

booted on ppc64be pseries9 VM, works fine so far. I assume ppc64le pseries9 VM will work the same.

Will test the powermac G5 next. I assume you did p8/p9 testing already? :-)