scheduler: make getVolumeLimits a function instead of a method

[SataQiu]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:
At first, I was confused.
I think the csiNode in the argument list of getVolumeLimits was redundant,
Because we can get the csiNode by some code like this:

csiNode, err := c.csiNodeInfo.GetCSINodeInfo(node.Name)

But when I got down to it, I realized that this function is only called by attachableLimitPredicate.
In attachableLimitPredicate function, we already computed the csiNode.
So it is efficient to use it directly, and there is no need to repeat the calculation.
Based on the above analysis, I think getVolumeLimits should be a normal function rather than a method.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[wgliang]

/lgtm

[SataQiu]

/assign @bsalamat

[SataQiu]

/assign @Huang-Wei

[SataQiu]

/assign @thockin Thanks :)

[draveness]

/uncc

[Huang-Wei]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Huang-Wei, SataQiu

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