kernel/vm: Decommit discarded pages of overcommitted caches.

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.

headers/private/kernel/vm/VMCache.h

@@ -135,7 +135,7 @@ public:
 	virtual	status_t			Adopt(VMCache* source, off_t offset, off_t size,
 									off_t newOffset);
 
-	virtual	status_t			Discard(off_t offset, off_t size);
+	virtual	ssize_t				Discard(off_t offset, off_t size);
 
 			status_t			FlushAndRemoveAllPages();
 
@@ -220,7 +220,7 @@ private:
 			void				_RemoveConsumer(VMCache* consumer);
 
 			bool				_FreePageRange(VMCachePagesTree::Iterator it,
-									page_num_t* toPage);
+									page_num_t* toPage, page_num_t* discarded);
 
 private:
 			int32				fRefCount;