[RFC] Revert "Disable idmapped mounts"

Hi.

With this merge request, I propose to revert this patch. Indeed, with a coworker, we are currently working on adding support for id map mounted volumes in runc. While working on this, we noticed the CI tests were failing with -EPERM being returned. After some debugging, we found out it was due to a patch applied in centos kernel: https://github.com/opencontainers/runc/pull/3717#issuecomment-1438429140

I am not here to tell you to absolutely revert this patch. But, at least, you should think about if you still need it today? Indeed, a lot of progress were made on ID map mounts since this patch was applied. Nowadays, several file systems, among them tmpfs and xfs, support ID map mount: https://elixir.bootlin.com/linux/v6.2/A/ident/FS_ALLOW_IDMAP https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=05e6295f7b5e05f09e369a3eb2882ec5b40fff20

Best regards and thank you in advance.