fix: support simultaneous create diff for same parent snapshot

[cosmoer]

In CreateDiff function, we wil create view snapshot base on parent snapshot,
view snapshot name is "<parent snapshot KEY>-parent-view",

so while, It does not support concurrent operations，other user will be return to the error that the snapshot already exists.

So, we add uniquePart() as a suffix.

[k8s-ci-robot]

Hi @cosmoer. Thanks for your PR.

I'm waiting for a containerd member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[dcantah]

LGTM