pnfs/blocklayout: fix end calculation in pnfs_num_cont_bytes

Use the number of pages in the pagecache mapping instead of the
number of pnfs requests which is only slightly related.

Reported-by: Weston Andros Adamson <dros@primarydata.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>