First pass at replacing the single global bounce pool with sub-pools that are

appropriate for different tag requirements.  With the former global pool,
bounce pages might get allocated that are appropriate for one tag, but not
appropriate for another, but the system had no way to distinguish between them.
Now zones with distinct attributes are created to hold pages, and each tag
that requires bouncing is associated with a zone.  New zones are created as
needed if no existing zones can meet the requirements of the tag.  Stats for
each zone are tracked via the hw.busdma sysctl node.

This should help drivers that are failing with mysterious data corruption.

MFC After: 1 week

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