Revert "VMAnonymousCache.cpp: changed page_index type to page_num_t"

This reverts commit f7176b0ee50d5367762d904a943a693b0a8e3e2f. Citing Ingo:
"off_t is the correct type to use for addressing pages in a cache/file,
which page_num_t should only be used for physical pages." I'll see how to
fix the GCC 4.7 warnings differently :)
Fixed typo in variable use.