This patch multithreads the main page reclaim function, shrink_cache().

This function used to run under pagemap_lru_lock.  Instead, we grab
that lock, put 32 pages from the LRU into a private list, drop the
pagemap_lru_lock and then proceed to attempt to free those pages.

Any pages which were succesfully reclaimed are batch-freed.  Pages
which were not reclaimed are re-added to the LRU.

This patch reduces pagemap_lru_lock contention on the 4-way by a factor
of thirty.

The shrink_cache() code has been simplified somewhat.

refill_inactive() was being called too often - often just to process
two or three pages.  Fiddled with that so it processes pages at the
same rate, but works on 32 pages at a time.

Added a couple of mark_page_accessed() calls into mm/memory.c from 2.4.
They seem appropriate.

Change the shrink_caches() logic so that it will still trickle through
the active list (via refill_inactive) even if the inactive list is much
larger than the active list.