cmd/derper: fix regression from bootstrap DNS optimization

The commit b9c92b90db earlier today
caused a regression of serving an empty map always, as it was
JSON marshalling an atomic.Value instead of the DNS entries map
it just built.

Change-Id: I9da3eeca132c6324462dedeaa7d002908557384b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/derper/bootstrap_dns.go

@@ -47,7 +47,7 @@ func refreshBootstrapDNS() {
 		}
 		dnsEntries[name] = addrs
 	}
-	j, err := json.MarshalIndent(dnsCache, "", "\t")
+	j, err := json.MarshalIndent(dnsEntries, "", "\t")
 	if err != nil {
 		// leave the old values in place
 		return