Add support for checking if a device is being used by a filesystem

[pwschuurman]

What type of PR is this?
/kind bug

What this PR does / why we need it:
Summary: If a PD block device is still being used by the filesystem (misconfiguration, mount namespace capture), return an error during NodeUnstage.

Currently the PDCSI driver performs an unmount on the globalmount during NodeUnstage, and success signals that ControllerUnpublish (GCE disk detach) can proceed. Normally successful mount is an indicator that there is no other consumer of the device on the OS. There are some sitautions where this may not be true:

1. Mount Namespace Capture: If a mount namespace is held open by a container process, due to an open file descriptor to /proc/<pid>/ns/mnt, a private mount point to the block device may exist in a container's mount namespace. Typically this only occurs if there is some other process that has root access to the OS and causes this to occur.
2. If there is some other user on the system which creates a private bind mount from the globalmount. The unmount and removal of the globalmount does not private to private bind mounts, resulting in a successful unmount in PDCSI, despite the device still being in use.

Which issue(s) this PR fixes:
Fixes #1657

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Check if volume's block device is still in use during NodeUnstage

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: pwschuurman

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[pwschuurman]

/assign @mattcary

[mattcary]

/lgtm

Good idea to test in e2e. Probably we should have done that for disable device...

[pwschuurman]

/cherry-pick release-1.13

[k8s-infra-cherrypick-robot]

@pwschuurman: #1658 failed to apply on top of branch "release-1.13":

Applying: Add support for checking if a device is being used by a filesystem
Using index info to reconstruct a base tree...
M	pkg/gce-pd-csi-driver/node.go
M	test/e2e/tests/single_zone_e2e_test.go
Falling back to patching base and 3-way merge...
Auto-merging test/e2e/tests/single_zone_e2e_test.go
Auto-merging pkg/gce-pd-csi-driver/node.go
CONFLICT (content): Merge conflict in pkg/gce-pd-csi-driver/node.go
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 Add support for checking if a device is being used by a filesystem
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

In response to this:

/cherry-pick release-1.13

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[pwschuurman]

@pwschuurman: #1658 failed to apply on top of branch "release-1.13":

Applying: Add support for checking if a device is being used by a filesystem
Using index info to reconstruct a base tree...
M	pkg/gce-pd-csi-driver/node.go
M	test/e2e/tests/single_zone_e2e_test.go
Falling back to patching base and 3-way merge...
Auto-merging test/e2e/tests/single_zone_e2e_test.go
Auto-merging pkg/gce-pd-csi-driver/node.go
CONFLICT (content): Merge conflict in pkg/gce-pd-csi-driver/node.go
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 Add support for checking if a device is being used by a filesystem
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

Cherry pick #1805

[pwschuurman]

Cherrypick for release-1.12: #1845