NFS4: Avoid migration loops

If a server returns itself as a location while migrating, the client may
end up getting stuck attempting to migrate twice to the same server.  Catch
this by checking if the nfs_client found is the same as the existing
client.  For the other two callers to nfs4_set_client, the nfs_client will
always be ERR_PTR(-EINVAL).
Signed-off-by: NBenjamin Coddington <bcodding@redhat.com>
Signed-off-by: NTrond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/nfs4client.c

@@ -817,6 +817,11 @@ static int nfs4_set_client(struct nfs_server *server,
 		goto error;
 	}
 
+	if (server->nfs_client == clp) {
+		error = -ELOOP;
+		goto error;
+	}
+
 	/*
 	 * Query for the lease time on clientid setup or renewal
 	 *