fix: Better check of RPC type requests (#6927)

guessIsRPCReq() considers all POST requests as RPC but doesn't
check if this is an object operation API or not, which is actually
confusing bucket forwarder handler when it receives a new multipart
upload API which is a POST http request.

Due to this bug, users having a federated setup are not able to
upload a multipart object using an endpoint which doesn't actually
contain the specified bucket that will store the object.

Hence this commit will fix the described issue.

cmd/generic-handlers.go

@@ -228,7 +228,8 @@ func guessIsRPCReq(req *http.Request) bool {
 	if req == nil {
 		return false
 	}
-	return req.Method == http.MethodPost
+	return req.Method == http.MethodPost &&
+		strings.HasPrefix(req.URL.Path, minioReservedBucketPath+"/")
 }
 
 func (h redirectHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {