Show the cause error in startup when directio is not supported (#9497)

This commit tries to create a file using direct i/o in the startup
so the server returns quickly and avoid cryptic other errors.

cmd/posix.go

@@ -175,7 +175,7 @@ func getValidPath(path string) (string, error) {
 	var rnd [8]byte
 	_, _ = rand.Read(rnd[:])
 	fn := pathJoin(path, ".writable-check-"+hex.EncodeToString(rnd[:])+".tmp")
-	file, err := os.Create(fn)
+	file, err := disk.OpenFileDirectIO(fn, os.O_CREATE, 0600)
 	if err != nil {
 		return path, err
 	}

cmd/storage-rest-server.go

@@ -747,7 +747,7 @@ func registerStorageRESTHandlers(router *mux.Router, endpointZones EndpointZones
 				}
 				hint := fmt.Sprintf("Run the following command to add the convenient permissions: `sudo chown %s %s && sudo chmod u+rxw %s`",
 					username, endpoint.Path, endpoint.Path)
-				logger.Fatal(config.ErrUnableToWriteInBackend(err).Hint(hint),
+				logger.Fatal(config.ErrUnableToWriteInBackend(err).Msg(err.Error()).Hint(hint),
 					"Unable to initialize posix backend")
 			}