Hard-linked files cannot be read and the link counts aren't correct.

[ktock]

Issue description

Currently we can't deal with hard-link.

Issue Reproduction

In the tar file of ubuntu:18.04 base layer(sha256:6958ba61ef42a87f438b918c94a12af7a64a415a23dbed8e364eb6af9bb0845a), there is a hardlink:

 hrwxr-xr-x       0/0             0 bin/uncompress ==> bin/gunzip

We can get the original file on CRFS:

$ cat /crfs/layers/gcr.io/reasonablek8s/ubuntu/18.04/0/bin/gunzip
#!/bin/sh
# Uncompress files.  This is the inverse of gzip.

# Copyright (C) 2007 Free Software Foundation
...

But the link counts aren't correct.

$ stat --print="%h\n" /crfs/layers/gcr.io/reasonablek8s/ubuntu/18.04/0/bin/gunzip
1
$ stat --print="%h\n" /crfs/layers/gcr.io/reasonablek8s/ubuntu/18.04/0/bin/uncompress
1

And we get an empty file from the hardlink.

$ file /crfs/layers/gcr.io/reasonablek8s/ubuntu/18.04/0/bin/uncompress
/crfs/layers/gcr.io/reasonablek8s/ubuntu/18.04/0/bin/uncompress: empty

Additionally, when we cat it, we get EIO.

The cause of issue

There are two causes:

• In TOCEntriys, link counts aren't recorded and the filesystem returns a static number 1 as link counts.
• When a filesystem looksup to a stargz file (with stargz.Reader.Lookup() and (*stargz.TOCEntry).LookupChild()), the hardlink isn't resolved.