vm_page no longer keeps a pointer to the vm_cache_ref but to the vm_cache.
This cleans the page handling when having to make an area copy-on-write;
regions belong to vm_cache_ref, pages to vm_cache.
Fixed some return types.
git-svn-id: file:///srv/svn/repos/haiku/trunk/current@9264 a95241bf-73f2-0310-859d-f6bbb57e9c96