SUNRPC: Add a backpointer from the struct rpc_cred to the rpc_auth

Cleans up an issue whereby rpcsec_gss uses the rpc_clnt->cl_auth. If we want
to be able to add several rpc_auths to a single rpc_clnt, then this abuse
must go.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

net/sunrpc/auth_unix.c

@@ -92,6 +92,7 @@ unx_create_cred(struct rpc_auth *auth, struct auth_cred *acred, int flags)
 		if (i < NFS_NGROUPS)
 		  cred->uc_gids[i] = NOGROUP;
 	}
+	cred->uc_base.cr_auth = &unix_auth;
 	cred->uc_base.cr_ops = &unix_credops;
 
 	return (struct rpc_cred *) cred;