BUG: add support for open_tree_attr(2)

[grooverdan]

This was added in https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c4a16820d90199409c9bf01c4f794e1e9e8d8fd8.

[poettering]

would love to see an update here. we started to disable libseccomp filtering in some systemd services now, because we want open_tree_attr(). for example see this:

https://github.com/systemd/systemd/pull/39144/files

[pcmoore]

@drakenclimber and I have talked about cutting a new release to update the syscall table (there are some minor fixes too), and ideally get into a more regular minor update cadence to better align with the introduction of new syscalls.

We'll try to get something out soon, I need to finish flushing the review/merge queue on the kernel now that the merge window is closed, but hopefully I can get back to this in a few days.