nfsd: rpc_peeraddr2str needs rcu lock

I'm not even sure cl_xprt can change here, but we're getting "suspicious
RCU usage" warnings, and other rpc_peeraddr2str callers are taking the
rcu lock.
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4callback.c

@@ -939,8 +939,10 @@ static int setup_callback_client(struct nfs4_client *clp, struct nfs4_cb_conn *c
 	}
 	clp->cl_cb_client = client;
 	clp->cl_cb_cred = cred;
+	rcu_read_lock();
 	trace_nfsd_cb_setup(clp, rpc_peeraddr2str(client, RPC_DISPLAY_NETID),
 			    args.authflavor);
+	rcu_read_unlock();
 	return 0;
 }