Add API equivalent to kernel's nonseekable_open

[alex]

Needed for #110

[nelhage]

Does this mean:

(1) Somehow bind nonseekable_open so that you can call into it from Rust in your open implementation, or
(2) Somehow implement support for directly binding any of the common C open functions: https://github.com/torvalds/linux/blob/v5.8/fs/open.c#L1326-L1372

You could imagine (2) working by removing open from the FileOperations trait, and adding an Open trait, and in addition to the open builder method, adding e.g. open_nonseekable() that binds to the kernel method.

[alex]

Either! Would need someone to look into it.

[alex]

The API for this is much more obvious with our new FileOperations API:

impl FileOperations for Foo {
    const SEEK: SeekFn<Self> = Some(open_nonseekable::<Self>);
}

[alex]

Err, I apparently don't have brain cells to rub together. open is a function on the trait, not a function-ptr in a const on it. Which means this just needs to be a function that open delegates to. Functions calling functions is good.