NFS: remove unused offset arg in nfs_pgio_rpcsetup

nfs_pgio_rpcsetup() is always called with an offset of 0, so we should be
able to drop the arguement altogether.

Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>