Overcommitted caches should only have commitments equal to the
number of pages they actually contain, so we should decommit
whenever pages are discarded.
This changes the API of VMCache::Discard to return an ssize_t
of the size of pages that were discarded (or a negative error on
failure.) Nothing checked the return value besides things in VMCache
itself, it appears; but it apparently never fails, so that's fine.
Also add asserts to Commit() that the new commitment at least
encompasses all pages the cache actually contains.
Augustin Cavalier
d00cb444a6