[cinder-csi-plugin] Protect OsInstances global map with mutex

[hemna]

What this PR does / why we need it:

The OsInstances map is accessed from both CreateOpenStackProvider (write) and GetOpenStackProvider (read) without synchronization. Although current usage is single-threaded at startup, this is a latent data race that would cause a runtime panic (concurrent map read and map write) if the driver is ever accessed concurrently.

This adds a sync.Mutex to guard read and write access to the OsInstances map.

Which issue this PR fixes(if applicable):
fixes #

Special notes for reviewers:
The mutex is intentionally a simple sync.Mutex rather than sync.RWMutex since write contention is negligible (only at startup). The lock is held for the minimum scope needed around map access, not around the full CreateOpenStackProvider call to avoid holding the lock during network I/O.

Release note:

[cinder-csi-plugin] Add mutex protection to OsInstances global map to prevent potential concurrent access panics.

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @hemna. Thanks for your PR.

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[kayrus]

/ok-to-test