feat: enable user namespace in kernel config

[mastersans]

This PR enables CONFIG_USER_NS in the kernel configuration.

As part of our effort to support running unit tests for different gadgets of Inspektor Gadget on various kernel versions, we're using Vimto with the kernel images provided by ci-kernel (CONFIG_USER_NS enabled fork currently). However, Inspektor Gadget relies on specific runners to generate events that are later captured by different gadgets for testing. During this process, we retrieve the inode number and attempt to create a runner associated with UserNsID. Without CONFIG_USER_NS enabled, we are unable to create this runner as expected.
Would it be possible to enable CONFIG_USER_NS in the main ci-kernels repository to address this issue?

Thanks!

reference: inspektor-gadget/inspektor-gadget#3631

[mauriciovasquezbernal]

What's the general thought on this folks? Are you willing to add configurations needed by other projects or should we fork this instead?

For instance, I just found that we also need CONFIG_FANOTIFY inspektor-gadget@3e1a378

[lmb]

In general I don't mind enabling features as long as they don't make the build significantly more complicated (due to dependencies) or bloat the kernel by too much. I'd probably not add virtual device drivers or filesystems though, since these kernels are meant to be used in a particular scenario.

Also, we wouldn't be able to give any guarantees as to keeping the feature set stable.

[dylandreimerink]

Hey @mastersans turns out that we now also need user namespaces in cilium/ebpf. I have merged #89, essentially an updated version of this PR, which was quicker then rebasing this one. So I will close this PR now in favor of #89.