[IMPROVEMENT] Add prep work for fixing the corrupted filesystem using fsck in KB

[derekbit]

Is your improvement request related to a feature? Please describe

fsck usually needs to be run to fix a corrupted filesystem. It is possible, however, that the incorrect fsck fix might cause serious data loss and corruption.
To mitigate the risk, a snapshot of the corrupted filesystem should be taken before any fix attempts are made. In case of an accident, the user can recover the volume. We can suggest the prep work in the KB.

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Additional context

https://serverfault.com/questions/786648/when-is-fsck-dangerous
https://docs.portworx.com/reference/cli/volume-check/

[longhorn-io-github-bot]

Pre Ready-For-Testing Checklist

• Where is the reproduce steps/test steps documented?
  The reproduce steps/test steps are at:

• Does the PR include the explanation for the fix or the feature?

• Which areas/issues this PR might have potential impacts on?
  Area: filesystem corruption
  Issues

• If labeled: require/doc Has the necessary document PR submitted or merged (including backport-needed/*)?
  The documentation issue/PR is at

#4440

[khushboo-rancher]

@derekbit Can you please help with the reproduction steps, so that we can test this?

[innobead]

@khushboo-rancher it's just extending the existing KB to make it clear for users about what they should pay attention to when doing fsck on a volume, so there is no needs to prepare any reproduce steps here.