sync filesystem cache to nas backend when unmount nas

[jdmelo]

…uccessfully

What type of PR is this?

/kind bug

What this PR does / why we need it:

Fixed losetup command output, Unmarshal failure issue

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

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

[linux-foundation-easycla]

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[huww98]

I'm not sure why we need this? Umount should flush dirty cache automatically. Sync() is global, it can be very expansive on a host with tens of volumes and hundreds of GiBs of dirty cache.