This is the first in a series of patches which tune up the 2.5
performance under heavy swap loads.

Throughput on stupid swapstormy tests is increased by 1.5x to 3x.
Still about 20% behind 2.4 with multithreaded tests.  That is not
easily fixable - the virtual scan tends to apply a form of load
control: particular processes are heavily swapped out so the others can
get ahead.  With 2.5 all processes make very even progress and much
more swapping is needed.  It's on par with 2.4 for single-process
swapstorms.


In this patch:

The code which tries to start mapped pages out on the active list
doesn't work very well.  It uses an "is it mapped into pagetables"
test.  Which doesn't work for, say, swap readahead pages.  They are not
mapped into pagetables when they are spilled onto the LRU.

So create a new `lru_cache_add_active()' function for deferred addition
of pages to their active list.

Also move mark_page_accessed() from filemap.c to swap.c where all
similar functions live.  And teach it to not try to move pages which
are in the deferred-addition list onto the active list.  That won't
work, and it's bogusly clearing PageReferenced in that case.

The deferred-addition lists are a pest.  But lru_cache_add used to be
really expensive in sime workloads on some machines.  Must persist.