fs: Do not return reservedBucket names in ListBuckets() (#3754)

Make sure to skip reserved bucket names in `ListBuckets()`
current code didn't skip this properly and also generalize
this behavior for both XL and FS.

cmd/browser-peer-rpc.go

@@ -111,7 +111,7 @@ func updateCredsOnPeers(creds credential) map[string]error {
 				secretKey:       serverCred.SecretKey,
 				serverAddr:      peers[ix],
 				secureConn:      globalIsSSL,
-				serviceEndpoint: path.Join(reservedBucket, browserPeerPath),
+				serviceEndpoint: path.Join(minioReservedBucketPath, browserPeerPath),
 				serviceName:     "BrowserPeer",
 			})