With both rootless and live restore enabled, there's some race condition
which causes the container to be `Unmount`ed before the refcount is
restored.
This makes sure we don't underflow the refcount (uint64) when
decrementing it.
The root cause of this race condition still needs to be investigated and
fixed, but at least this unflakies the `TestLiveRestore`.
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>
294fc9762e