NFSv4: Fix a memory leak in nfs4_discover_server_trunking

When we assign a new rpc_client to clp->cl_rpcclient, we need to destroy
the old one.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: Chuck Lever <chuck.lever@oracle.com>
Cc: stable@vger.kernel.org [>=3.7]