Implement Metadata:from_statx for Linux MetadataExt trait

[asder8215]

This PR implements the Metadata::from_statx for Linux MetadataExt ACP. You can create a std::fs::Metadata from a *const c_void populated by the statx syscall.

The tracking issue for this ACP is here.

I had a couple of questions regarding to what Josh said in the ACP:

We'd also be interested in adding a constant for "the statx flags std normally requests", so that code wanting to request everything std knows about can use those.
^I wanted a little bit more clarity on what Josh meant by this.

• How should I go about adding in flags/masks for populating the Metadata struct with the *const c_void pointer containing the statx info? I'm assuming it's possible that not all of the fields from our statxbuf will be submitted or given to Metadata stat fields (since I understood that this function is supposed to be flexible), so we risk accessing something not initialized?
• Should this function return a Result<Metadata>? Should I be checking whether the given *const c_void is a null pointer or not, and returning an Error if it is?

Documentation for this function is a WIP. There's probably a lot more I have to say for this function.

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r? @jhpratt

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[joshtriplett]

I had a couple of questions regarding to what Josh said in the ACP:

We'd also be interested in adding a constant for "the statx flags std normally requests", so that code wanting to request everything std knows about can use those.
^I wanted a little bit more clarity on what Josh meant by this.

* How should I go about adding in flags/masks for populating the `Metadata` struct with the `*const c_void` pointer containing the statx info? I'm assuming it's possible that not all of the fields from our `statxbuf` will be submitted or given to `Metadata` stat fields (since I understood that this function is supposed to be flexible), so we risk accessing something not initialized?

You can provide a single public const STATX_MASK, of type c_int, for the appropriate mask. It's the caller's responsibility to make sure std doesn't encounter uninitialized data.

* Should this function return a `Result<Metadata>`? Should I be checking whether the given `*const c_void` is a null pointer or not, and returning an `Error` if it is?

It's an unsafe function; you have to pass it a valid pointer. We could panic if we get null, as a precaution, but I'm not sure we should; I think we should just assume the pointer is valid. The function should return a Metadata.

[asder8215]

r? libs

[Darksonn]

I was planning to wait for #154981 before implementing this to avoid this clause.

View changes since the review

[asder8215]

Got you. I think the order doesn't matter right now since this function would automatically have musl support once #154981 lands as the PR updates the cfg_has_statx macro. The only thing that needs to be updated is the documentation for this function, which I'm down to keep my eyes on #154981 to see it get merged in.