Some adjustments to global dirty page accounting.

Previously, dirty page accounting counted all dirty pages.  Even dirty
anonymous pages.  This has potential to upset the throttling logic in
balance_dirty_pages().  Particularly as I suspect we should decrease
the dirty memory writeback thresholds by a lot.

So this patch changes it so that we only account for dirty pagecache
pages which have backing store.  Not anonymous pages, not swapcache,
not in-memory filesystem pages.

To support this, the `memory_backed' boolean has been added to struct
backing_dev_info.  When an address space's backing device is marked as
memory-backed, the core kernel knows to not include that mapping's
pages in the dirty memory accounting.

For memory-backed mappings, dirtiness is a way of pinning the page, and
there's nothing the kernel can to do clean the page to make it freeable.

driverfs, tmpfs, and ranfs have been coverted to mark their mappings as
memory-backed.

The ramdisk driver hasn't been converted.  I have a separate patch for
ramdisk, which fails to fix the longstanding problems in there :(

With this patch, /bin/sync now sends /proc/meminfo:Dirty to zero, which
is rather comforting.