Avoid duplicated snapshotting, when CSI is enabled

[blackpiglet]

Do not take snapshot for PV to avoid duplicated snapshotting, when CSI feature is enabled.

Signed-off-by: Xun Jiang jxun@vmware.com

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Fixes #4758

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[reasonerjt]

This is problematic because the PV may not be provisioned by CSI driver.

[blackpiglet]

Tried to add some annotations on PV when executing VolumeSnapshot BackupItemAction in backup process, so PV can tell whether it's handled by CSI plugin, but it turned out Velero backup resources according to resources group's name alphabetical order.
That leads to the result that PV is backup before VolumeSnapshot, so it cannot judge by the annotations.
After reading the CSI plugin's PVC BackupItemAction related code, I think the current code should work. CSI plugin doesn't handle PV if its pv.Spec.PersistentVolumeSource.CSI is nil.
I think it should be safe to tell whether this PV should be handled by CSI plugin with this condition:
features.IsEnabled(velerov1api.CSIFeatureFlag) && pv.Spec.CSI != nil

https://github.com/vmware-tanzu/velero-plugin-for-csi/blob/main/internal/backup/pvc_action.go#L82-L85

[reasonerjt]

lgtm