net :sunrpc :clnt :Fix xps refcount imbalance on the error path

rpc_clnt_add_xprt take a reference to struct rpc_xprt_switch, but forget
to release it before return, may lead to a memory leak.

Signed-off-by: Lin Yi <teroincn@163.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>