Fix the calculation that determines uz_maxpages. It was off for large zones.

Fix the calculation that determines uz_maxpages.  It was off for large zones.
Fortunately we have no large zones with maximums specified yet, so it wasn't
breaking anything.

Implement blocking when a zone exceeds the maximum and M_WAITOK is specified.
Previously this just failed like the old zone allocator did.  The old zone
allocator didn't support WAITOK/NOWAIT though so we should do what we
advertise.

While I was in there I cleaned up some more zalloc logic to further simplify
that code path and reduce redundant code.  This was needed to make the blocking
work properly anyway.