mpsc: ensure try_reserve error is consistent with try_send

[alanbriolat]

Fixes #4118.

Motivation

try_reserve() and try_reserve_owned() of tokio::sync::mpsc::Sender are inconsistent with try_send() in the error they return, signalling Full instead of Closed for a closed channel. If other code relies on this error as documented, it might forever retry sending to a closed channel.

Solution

I copied the handling of the TryAcquireError from try_send() to try_reserve() and try_reserve_owned(); as far as I can tell, this is probably what it should have been in the first place when try_reserve() was introduced by 672be92, and then try_reserve_owned() just copied try_reserve() behaviour.

[Darksonn]

Thanks!