multipage pagecache writeout

These patches:

	http://www.zip.com.au/~akpm/linux/patches/2.5/2.5.6-pre1/mpio-10-biobits.patch
	http://www.zip.com.au/~akpm/linux/patches/2.5/2.5.6-pre1/mpio-20-core.patch
	http://www.zip.com.au/~akpm/linux/patches/2.5/2.5.6-pre1/mpio-30-ext2.patch

implement multipage writeout from the pagecache. These patches require the allocate-on-flush patches. The dalloc-30-ratcache patch is not a requirement for the mpio series. But is recommended for balls-to-the-wall how-fast-can-it-go testing.

Pages from the pagecache are given a disk mapping, are assembled into large BIOs (up to half a megabyte) and these BIOs are injected direct into the request layer.

These pages never have attached buffer_heads. The buffer layer is completely bypassed for all write(2) data. As is, to some extent, the request merging layer.

This patch should bypass the lru_list_lock contention problem, and the ZONE_NORMAL-full-of-buffer_heads bug. (Well, this may require multipage reads, too).

Future work includes:

- Implement buffer_head-less block_truncate_page().

- multipage reads.

A bit of a no-brainer, but first the current readahead code needs a big shakeout.

Two additional patches are available:

http://www.zip.com.au/~akpm/linux/patches/2.5/2.5.6-pre1/tuning-10-request.patch

- The get_request starvation fix for 2.5.

This patch also increases the request queue by a lot. Which implies that we can have as much as 512 megabytes of I/O underway per device. This may sound excessive, but the locked- and dirty-page accounting in the delalloc patch only permits this to happen if the machine is large enough to cope with it.

http://www.zip.com.au/~akpm/linux/patches/2.5/2.5.6-pre1/tuning-20-ext2-preread-inode.patch
- Pull the backing block for a new ext2 inode into the buffercache when the inode is created. This fixes a significant throughput problem with many-file writeout, where the writer is continually interrupted by having to perform reads.