NFS: drop unused nfs_direct_req bytes_left

Now that we're calculating how large a remaining IO should be based
on the current request's offset, we no longer need to track bytes_left on
each struct nfs_direct_req.  Drop the field, and clean up the direct
request tracepoints.

Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>