fix: storage plugin processes leak.

[yuanqijing]

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fix #7925

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Fixes #(issue)

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[sseago]

@yuanqijing We actually don't need to initialize a new plugin manager at all in FinalizeBackup -- we can pass in the one we already initialized in the backup finalizer controller. I'm testing an alternate fix that does that and will post it if it works.

[sseago]

@yuanqijing I've submitted #8012 which passes in the already-initialized plugin manager from the finalizer controller, so we don't need to spawn a new plugin process for get/put volume info. I've tested this locally and it resolves the leak.

[sseago]

I'm going to go ahead and close this PR since #8012 has been merged which also fixes it.