RPC: allow call_encode() to delay transmission of an RPC call.

 Currently, call_encode will cause the entire RPC call to abort if it returns
 an error. This is unnecessarily rigid, and gets in the way of attempts
 to allow the NFSv4 layer to order RPC calls that carry sequence ids.

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