tools/profile: add support for PID-namespacing

[lucab]

This adds translation logic to perform PID remapping across PID-namespaces.
It is now possible to profile a target process from within a nested PID-namespace (e.g. in a container).

[lucab]

/cc @yonghong-song for review.

[yonghong-song]

Can we add an option for this new functionality, something like '--with-pid-namespace'? Note that with '--with-pid-namespace', the command line arguments like pid/tid should be the one in the pid namespace to make the tool work.
With introducing new option, please add a few examples in the example file and update the man page as well.

[lucab]

@yonghong-song thanks for the feedback! I've happily added a note to the examples file, but if possible I'd push back against adding a CLI option.

This is really a core bugfix more than a new feature: the profile tool is already always running in some kind of pid-namespace (either the top-level host one, or a nested children down the chain), but currently it produces wrong results for anything other than the host namespace, without giving any clues to the user.

Specifically, when running in a children namespace it will happily target a host-PID which by chance shares the same number as a current-children-ns-PID.
The resulting stacktrace will have some proper content with meaningless addresses (because they belong to an unexpected process) and symbol resolution will be incorrect (same reason).

I think a --with-pid-namespace flag could be added in the future for some new advanced feature (e.g. targeting arbitrary non-self namespaces), but this PR instead is only about fixing a bug in the current eBPF code.

[yonghong-song]

Okay, @lucab Your argument above indeed makes sense. Without this symbolization will be incorrect in child pid namespace.

[lucab]

@yonghong-song good to hear, and thanks for taking the time to review this!