Improve Kselftest coverage with fault injection

[l0kod]

Fault injection enables to trigger kernel errors (e.g. when calling kmalloc()) and then test rare cases.

It would be useful to improve kernel test coverage by extending currents test cases with kernel fault injections (e.g., thanks to /proc/self/task/<tid>/fail-nth). It would be great to be able to deterministically test all Landlock's related code.

Example: iommufd_fail_nth.c.