NFSv4: Fix the underestimation of delegation XDR space reservation

Account for the "space_limit" field in struct open_write_delegation4.

Fixes: 2cebf828

 ("NFSv4: Fix the underestimate of NFSv4 open request size")
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>