fix off-by-one error in range check of Backlog::new

[fkm3]

What does this PR do

Makes it possible to call Backlog::new(libc::SOMAXCONN) without error.

In the case where I encountered this, a constant is passed to Backlog::new
and it just happens to be equal to SOMAXCONN on some platforms.

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[fkm3]

The CI failure looks clearly unrelated

error: missing documentation for a function
   --> src/features.rs:95:5
    |
95  |     pub fn test_parsing_kernel_version() {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |

[SteveLauC]

Thanks for catching it!

CI failure is not related, I will give it a fix.

[SteveLauC]

Hi, CI fixed in #2501, please rebase your branch:)

[fkm3]

Thanks! Rebased and rerunning checks now

[SteveLauC]

Thanks for the patch!