Unable to restore containers checkpointed with `-nvproxy` and `-nvproxy-docker` about gvisor HOT 3 CLOSED

This must have been broken by cf9d55b.

validateOpenFDs() is a security check that happens right before the application starts which checks that the sandbox process does not have any open directory FDs. This is to prevent leaking directory FDs to the sandbox. Because with directfs, sandbox has access to openat(2) syscall and we don't want a compromised sandbox to be able to walk on these directories.

Notes:
prepareMounts() initializes the gofer connection:

It is called after seccomp filters are installed in the normal case (via runsc start), but it is called before seccomp filters are initialized in the restore case. When the connection is initialized, it donates a directory FD to the sandbox to use. This is what validateOpenFDs() is complaining about.

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@luiscape FYI we fixed another GPU checkpoint/restore bug (b373c8e) and added a GPU checkpoint restore test (1214e28). So hopefully we don't break you again.

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@ayushr2 awesome. Thanks a lot. It's been working well so far. I'll send you a detailed update this week.

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