Request: add error handling in pkg/pwalk

[zeroalphat]

When I tried to start a container with SELinux labeling for volume, I got the following error and could not start a container.

$ docker run --log-driver=journald -d --name=kubelet --read-only --network=host \
--uts=host --pid=host --privileged \
--tmpfs=/tmp --volume=/var/lib/kubelet:/var/lib/kubelet:rshared,z \
quay.io/cybozu/kubernetes:1.25.6.1 kubelet

docker: Error response from daemon: 
error setting label on mount source '/var/lib/kubelet': 
lstat /var/lib/kubelet/seccomp/2/pwalk-test-1270530707/d-2753637893/f-1857926114: no such file or directory.

I think this error was returned from pkg/pwalk in opencontainers/selinux.
I found that the error lstat no such file or direcotry is returned if a file is deleted while filepath.Walk of the pwalk function is retrieving the list of files in a directory.
In my environment, it is possible that files under the target directory are frequently deleted during the process of SELinux labeling.
I think it is undesirable behavior to return an error, even if it is a specific environment.
How about adding error handling to make it work in such environment?
https://github.com/opencontainers/selinux/blob/main/pkg/pwalk/pwalk.go#L88C2-L91

Environments.

• docker version: v20.10.18
• opencontainers/selinux version: v1.8.0

To Reproduce.

1. Add a function for testing to pwalk_test.go
2. Create a direcotry and files for testing using the prepareTest function
3. Execute the Walk function on the created direcotry.

err = Walk(dir,
		func(p string, i os.FileInfo, e error) error {
			return nil
		})

4. Deletes all files under a particular directory during execution of the pwalk.Walk function.
5. Get an error lstat <directory/file name> no such file or directory

delete pwalk-test-2649712757/d-1782752606 directory
--- FAIL: TestRemoveFile (0.01s)
    pwalk_test.go:55: lstat pwalk-test-2649712757/d-1782752606/d-2791693006: no such file or directory
FAIL
exit status 1
FAIL    github.com/opencontainers/selinux/pkg/pwalk     0.011s

[rhatdan]

@kolyshkin PTAL

[kolyshkin]

Should be addressed by #204. I was able to reproduce the issue and add a test case, which fails before the fix.