fs: Preserve the max_buf_size when cloning a File

[martin-g]

Motivation

File::try_clone() should preserve the max_buf_size of the origin, otherwise the cloning would use the default (tokio::io::blocking::DEFAULT_MAX_BUF_SIZE)

Solution

Set the max_buf_size of the cloning to the one used by the origin.

[martin-g]

Adding a new IT test in tokio/tokio/tests/fs.rs requires to add a public getter for File::max_buf_size. Is that OK ?
Or should I add a unit test ? There are no such for the moment:

tokio/tokio/src/fs/file.rs

Line 984 in 9f59c69

mod tests;

[Darksonn]

I'm okay with adding a getter.

[Darksonn]

We usually end up with one changelog entry per PR. Could you make two PRs so that we have one for adding a new public method, and another for fixing try_clone()?

[martin-g]

Sure! I will keep this PR for the second part and rebase it once the (to-be-created) PR for the getter is merged.

[Darksonn]

Thanks.