Fix block ciphers + HMAC_SHA1_ETM

[Barre]

Due to an incorrect specified array length, any combinaison of a cipher using block.rsand HMAC_SHA1_ETM such as:

let mut config = client::Config::default();
config.preferred.cipher = &[cipher::AES_128_CTR];
config.preferred.mac = &[mac::HMAC_SHA1_ETM];

Would fail during packet auth.

[...]
[2024-06-07T12:49:14Z DEBUG russh::cipher] reading, clear len = 276
[2024-06-07T12:49:14Z DEBUG russh::cipher] read_exact 280
[2024-06-07T12:49:14Z DEBUG russh::cipher] read_exact done
[2024-06-07T12:49:14Z TRACE russh::client] disconnected
[2024-06-07T12:49:14Z DEBUG russh::client] disconnected: Error(PacketAuth)
[2024-06-07T12:49:14Z TRACE mio::poll] deregistering event source from poller
[2024-06-07T12:49:14Z DEBUG russh::client] drop session
[2024-06-07T12:49:14Z DEBUG russh::client] drop handle

This PR fixes that.

[Eugeny]

Thank you!

@all-contributors please add @Barre for code

[allcontributors]

@Eugeny

I've put up a pull request to add @Barre! 🎉

[Barre]

Thank you!

@all-contributors please add @Barre for code

Thank you as well, love the work you did with russh.

[allcontributors]

@Barre

@Barre already contributed before to code