refactor(udp): define const for receive buffer size

[mxinden]

Follow up to #1707 (comment).

Reading multiple datagrams in single Socket::recv call will happen with #1708.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.97%. Comparing base (78919a5) to head (0a878b3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1709   +/-   ##
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  Coverage   92.97%   92.97%           
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  Files         120      120           
  Lines       37342    37342           
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  Hits        34718    34718           
  Misses       2624     2624           

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[larseggert]

Does quinn-udp maybe already define a constant for max. GSO size?

[mxinden]

quinn-udp exposes the system dependent GSO and GRO max:

https://github.com/quinn-rs/quinn/blob/9611c5ef6932781eb84bc775e7f008474531b20e/quinn-udp/src/unix.rs#L166-L183

quinn itself relies on the system dependent GRO max segments and the recvmmsg batch size to determine its receive buffer size.

https://github.com/quinn-rs/quinn/blob/9611c5ef6932781eb84bc775e7f008474531b20e/quinn/src/endpoint.rs#L685-L690

I suggest we introduce this logic once we introduce proper GSO and GRO support (#1693). I see this pull request and #1708 as bug fixes only.