NFS: Clean up and fix page zeroing when we have short reads

The code that is supposed to zero the uninitialised partial pages when the
server returns a short read is currently broken: it looks at the nfs_page
wb_pgbase and wb_bytes fields instead of the equivalent nfs_read_data
values when deciding where to start truncating the page.

Also ensure that we are more careful about setting PG_uptodate
before retrying a short read: the retry will change the nfs_read_data
args.pgbase and args.count.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>