[PATCH] fix layout of mapped files

[PATCH] fix layout of mapped files

If you create a shared mapping of a sparse file, dirty it
and then run msync, all the file's blocks are laid out
backwards.

This is because filemap_sync puts the lowest-index page at
mapping->dirty_pages.prev and the highest at mapping->dirty_pages.next.

I think that by walking the dirty pages list in ascending file
offset order as we instantiate their disk mappings we will generally
get better layout.