kernel/vm: Correct virtual function declarations.

The base VMCache class changed to the generic_ types with their
introduction in in *2011* (435c43f591),
but these classes were never properly adapted. These functions should not
be called here (they panic() -- but the base class only returns B_ERROR,
so that is a difference at least.)

Found by Clang's -Woverloaded-virtual.

headers/private/kernel/vm/VMCache.h

@@ -147,7 +147,7 @@ public:
 	virtual	bool				HasPage(off_t offset);
 
 	virtual	status_t			Read(off_t offset, const generic_io_vec *vecs,
-									size_t count,uint32 flags,
+									size_t count, uint32 flags,
 									generic_size_t *_numBytes);
 	virtual	status_t			Write(off_t offset, const generic_io_vec *vecs,
 									size_t count, uint32 flags,