fs: implement tokio::fs::create_dir[_all] via io_uring

[tahmid-23]

io_uring + mkdirat()

As opposed to std's implementation of recursive directory creation, this does not use a recursive function to create parent directories. Instead, it iterates over positions of / (the unix / linux path separator), first failing to create parent directories until success, and then creating all children.

This prevents us from allocating a boxed future for each recursive function call.
It does have the minimal overhead of needing to find the / positions an additional time in the second forward iterator. We could save this by allocating a Vec of / positions; I don't necessarily know what's better.

[martin-g]

The build fails with:

error[E0277]: the `?` operator can only be applied to values that implement `std::ops::Try`
  --> tokio/src/fs/create_dir.rs:60:12
   |
60 |         if driver_handle.check_and_init(io_uring::opcode::MkDirAt::CODE)? {
   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the `?` operator cannot be applied to type `impl std::future::Future<Output = std::result::Result<bool, std::io::Error>>`
   |
   = help: the trait `std::ops::Try` is not implemented for `impl std::future::Future<Output = std::result::Result<bool, std::io::Error>>`
help: consider `await`ing on the `Future`

[martin-g]

Why not use Path::parent() ?

[martin-g]

And here you can use Path::components()

[martin-g]

metadata(path).await? may hide the original error (the ignored one in Err(_))

Suggested change

	Err(_) if crate::fs::metadata(path).await?.is_dir() => Ok(true),
	Err(ref e) if e.kind() == io::ErrorKind::AlreadyExists => {
	if crate::fs::metadata(path).await?.is_dir() {
	Ok(true)
	} else {
	Err(e)
	}
	}

something like this should be better

[martin-g]

Same as above

[martin-g]

This cfg guard is not really needed.
This module is already conditionally used with cfg_io_uring! in mod.rs

[martin-g]

Suggested change

	use std::io;
	use std::io::Error;
	use std::io::{self, Error};

[martin-g]

Those seem copied from one of the other tests.
Let's extract them into a helper module.