Pool capacity is not checked in ControllerExpandVolume

[plaffitt]

Describe the problem/challenge you have
I've submitted a PR (#656) that revert the (ref)quota of a volume if its (ref)reservation couldn't been expanded in order to fix the issue (#649) that render a volume having different values for its (ref)quota and (ref)reservation. While reverting fixes the issue, it is also the least desirable option, however NodeExpandVolume has to handle this case anyway.

Describe the solution you'd like
A way to prevent this to happen (most of the time) would be to implement a check in ControllerExpandVolume, we should check that the additional capacity doesn't exceed the currently available capacity, and fail with a ResourceExhausted error code when it does. And that way, the ZfsVolume wouldn't even be updated with the new (invalid) size.

Anything else you would like to add:
Maybe we can read the currently available capacity from the corresponding CSIStorageCapacity object, or maybe even better: we could reuse the code from the controller.GetCapacity function.

[Abhinandan-Purkait]

@plaffitt Would you be interested in contributing this. We already have discussed this on your previous PR. Below is an example how we can probably do this, but bear in mind we also need to keep in mind the thin scenarios. Thanks!

https://github.com/openebs/zfs-localpv/compare/add_check_controller?expand=1

[plaffitt]

@Abhinandan-Purkait For now I'm unfortunately too busy on other projects. I cannot promise you anything but I can try to look at in a few months if nobody handled it already.

[Abhinandan-Purkait]

@plaffitt Sure, I meant no rush, I asked you because you made a significant contribution so thought might be interested after we discussed on the previous PR. Thanks for your contributions!