pnfs/blocklayout: set up layoutupdate_pages properly

We need to replace the __be32 with a void pointer to do proper arithmentics
on the virtual addresses so that we can get the right page pointers.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>