reject PVCs with volumeMode block

[fungaren]

As explained in Kubernetes CSI Developer Documentation, we should check for the VolumeCapabilities as Kubernetes will not call ValidateVolumeCapabilities before CreateVolume. So we should check if the volumeMode is Block and if so, reject it.

Close #604

$ kubectl get pvc
NAME                  STATUS    VOLUME                                     CAPACITY   ACCESS MODES   STORAGECLASS          AGE
juicefs-root-bucket   Bound     pvc-71eea69d-cafc-47ca-9477-8c1b0b533ba0   10Pi       RWX            juicefs-root-bucket   11s
should-fail-pvc       Pending

E0321 07:08:29.886388       7 driver.go:96] GRPC error: rpc error: code = InvalidArgument desc = Volume Capabilities not fully supported

[zwwhdls]

Thanks for your contribution a lot! @fungaren

Can you fix the unit test?

[fungaren]

@zwwhdls Done. By the way, it is supposed to upgrade to gomonkey/v2 because it compiles failed in my machine. And must disable inlining by flag -gcflags=-l (below go1.10) or -gcflags=all=-l (go1.10 and above) to run the tests. And don't forget to run go mod tidy as well.

[zwwhdls]

@fungaren , thanks! Which version of golang do you use? And will you fix it?