SUNRPC: Add xdr_stream::rqst field

Having access to the controlling rpc_rqst means a trace point in the
XDR code can report:

 - the XID
 - the task ID and client ID
 - the p_name of RPC being processed

Subsequent patches will introduce such trace points.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>