SUNRPC allow for unspecified transport time in rpc_clnt_add_xprt

If the supplied argument doesn't specify the transport type, use the
type of the existing rpc clnt and its existing transport.

Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>