NFSv4.1: rearrange ->doio args

This will make it possible to clear the lseg pointer in the same
function as it is put, instead of in the caller nfs_pageio_doio().

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