nfs: rename members of nfs_pgio_data

Rename "verf" to "writeverf" and "pages" to "page_array" to prepare for
merge of nfs_pgio_data and nfs_pgio_header.

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