NFS: remove user_addr, user_count, and pos from nfs_direct_req

Make the user_addr, user_count, and pos parameters explicit to the
scheduler routines, and remove the fields from nfs_direct_req.  The
iovec API will be passing in a series of these, not just one set.

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