Allow creation of snapshot content if pvc finalizer exists, even if pvc is marked for deletion.

[RaunakShah]

What type of PR is this?
/kind bug

What this PR does / why we need it:
Currently there's a timing issue in createSnapshotContent which does not proceed if a PVC is marked for deletion, irrespective of whether the PVCFinalizer already exists on the PVC.
The result is that the PVC will remain in Terminating state and VolumeSnapshot.ReadyToUse will be false infinitely.

$ kubectl get pvc -A
NAMESPACE               NAME       STATUS        VOLUME                                     CAPACITY   ACCESS MODES   STORAGECLASS                                                   AGE
csi-mock-volumes-6032   test-pvc   Terminating   pvc-d2b09880-89ea-42a0-9896-c1159b35f349   2Gi        RWO            csi-mock-volumes-6032-csi-mock-csi-mock-volumes-6032-scczwbv   4m
$ kubectl get volumesnapshot -A
NAMESPACE               NAME             READYTOUSE   SOURCEPVC   SOURCESNAPSHOTCONTENT   RESTORESIZE   SNAPSHOTCLASS                                              SNAPSHOTCONTENT   CREATIONTIME   AGE
csi-mock-volumes-6032   snapshot-ssd5d   false        test-pvc                                          csi-mock-volumes-6032-csi-mock-csi-mock-volumes-6032-vsc                                    4m3s

This change returns success if the PVC Finalizer exists. If it doesnt exist, we check for deletionTimestamp before adding the finalizer.

Steps to reproduce:

1. Create Snapshot and PVC together
2. Delete PVC before Snapshot.ReadyToUse is true

Which issue(s) this PR fixes:

Fixes #349

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Bug fix to allow creation of snapshot content if pvc finalizer exists, even if pvc is marked for deletion.

[RaunakShah]

/assign @xing-yang

[xing-yang]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: RaunakShah, xing-yang

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