updated version of radix-tree pagecache

I've just uploaded an updated version of Momchil Velikov's patch for a scalable pagecache using radix trees. The patch can be found at:

	ftp://ftp.kernel.org/pub/linux/kernel/people/hch/patches/v2.4/2.4.17/linux-2.4.17-ratpagecache.patch.gz

	ftp://ftp.kernel.org/pub/linux/kernel/people/hch/patches/v2.4/2.4.17/linux-2.4.17-ratpagecache.patch.bz2

It contains a number of fixed and improvements by Momchil and me.

The basic advantage over the old version (besides the fixes :)) is that the radix tree implementation is now independand of struct page / struct address_space and thus can easily be used in other code.

=== Changelog ===

Momchil Velikov:

- It was possible to return a PG_locked page to the buddy allocator with a subsequent oops, if the call to rat_insert in __add_to_page_cache failed. Thus the functions is changed as to avoid modifying the pages before rat_insert was successful. Somewhat paranoid, I changed add_page_cache_locked too.
- shmem_writepage was causing an infinite looping deadlock, when a couple of processes was yielding for kswapd, _including kswapd itself_.
- Initialized swapper_space. On some architectures the spinlock is initilized to 0 on some to 1, who knows maybe there are/will be others. I have no idea why this didn't break the test on OSDL's 4- and 8-way boxes.

Me:

- moved rat.c from mm/ to lib/.
- new structure: rat_root containing root-node, height and gfp_mask.
- changed rat_* arguments to struct rat_root * and void *.
- change struct page * arguments to void *.
- moved all declarations in rat.h that are not public to rat.c
- replaced page_cache_init() by ratcache_init() in rat.c.
- rat_node slab handling moved to rat.c
- in swap_state.c removed 0/NULL initializers that aren't needed.
- replaced __find_get_page/__find_lock_page with non-prefixed versions.
- added kdoc-style comments to rat.c.
- fixed up whitespaces in function declarations to math Linux style.