zonelist cache performance

improve performance with changes to the zonelist cache. But I don't claim to have tested on an extensive list of platforms and/or benchmarks, so I was hoping for feedback.

The proposal is, essentially, to rip out the zonelist cache and
replace it with a single int which caches the index i into the zonelist[i] for the most recently allocated page. Any future attempt to allocate a page starts at zonelist[i], with a failure reverting to a full scan of all zonelists. The theory is that this will succeed most of the time, and as such it should be as lightweight as possible. zonelist_cache is only fast if zonelist[0] has a free page.

In the context of fake numa where numa=fake=<n> has a large <n>,
zonelist[0] may well fill up quickly yet the system still has a lot of free memory. As such, starting the allocation at zonelist[i] seems faster. In the event the allocation fails, we do a slow, full search, so this only works if that's the rare case.

As to the performance improvement, it improved kernbench by 6% with
numa=fake=64 and 2% without fake numa.

Thanks,
-- Ethan

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