After we refactor the magic mount and always mount folder as tmpfs,
we can easily support deleting files or folders now. We recognize
dummy devices with major number 0 and minor number 0 as an indicator
for removing files and folders. This indicator is borrowed from
overlayfs.
Mirror was previously used for accessing the original file during
magic mount when we are using a tmpfs to cover the target. However,
since we introduce atomic mount, we switch all tmpfs mount in
worker and then move to the target at once. It means that we can
still access the original file when we are constructing the tmpfs
mount point. Thus we no longer need mirror.
