Fix partial cache tail latency by correcting the cache chunk size calc

[cemakd]

What type of PR is this?
/kind bug

What this PR does / why we need it:
The GKE Data Cache was exhibiting extreme tail latency on partially
cached volumes, performing significantly worse than non-cached volumes.
Initial investigation into LVM cache policies and atime-induced
write-back thrashing did not reveal the root cause.

A series of fio benchmarks comparing cached and uncached volumes
uncovered a bug where the LVM cache was being configured with a
massive 1 GB chunk size. This was traced to a failure in the
fetchPvSizeGiB function in cache.go. The pvs command syntax was
malformed, and a subsequent string parsing error on its empty output
caused the driver to fall back to the default maxChunkSize of 1 GB.

This 1 GB chunk size led to extreme "read and write amplification," where a
single 4KB cache miss would trigger an inefficient 1 GB read from the
backing disk, explaining the catastrophic per-miss penalty.

This change corrects the pvs command syntax and the string parsing
logic within the driver. With this fix, the cache is now correctly
configured with a reasonable chunk size (e.g., 384 KiB), which resolves
the extreme read and write amplification and restores performance to expected
levels.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Fix partial data cache tail latency by correcting the cache chunk size calc

[mattcary]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cemakd, mattcary

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[sunnylovestiramisu]

Waiting for e2e testing edge cases confirmation

/hold

[k8s-ci-robot]

@cemakd: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-gcp-compute-persistent-disk-csi-driver-e2e-arm	648f174	link	false	/test pull-gcp-compute-persistent-disk-csi-driver-e2e-arm
pull-gcp-compute-persistent-disk-csi-driver-e2e-windows-2019	648f174	link	false	/test pull-gcp-compute-persistent-disk-csi-driver-e2e-windows-2019

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[cemakd]

/unhold e2e verification complete, the new chunk size is 2,368 KiB (or ~2.31 MiB) on a cluster with data-cache-count=6

[cemakd]

/cherry-pick release-1.21

[k8s-infra-cherrypick-robot]

@cemakd: new pull request created: #2177

In response to this:

/cherry-pick release-1.21

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