appear: when freeing a modified page, it wouldn't have a cache
anymore, but set_page_state_nolock() depended on it.
* To work around this, I added a vm_page_free() function, which the
caches that free modified pages have to call (but others may, too).
It will correctly maintain the sModifiedTemporaryPages counter in case
the cache has already been removed.
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@23318 a95241bf-73f2-0310-859d-f6bbb57e9c96