socket: fix listen(2) on an already listening socket

[glebius]

Summary

Fixes: 141fe2dceeaeefaaffc2242c8652345a081e825a

[markj]

Oops, thanks.

Will you add a test case? I'll write one if not.

[glebius]

In D35669#808847, @markj wrote:

Oops, thanks.

Will you add a test case? I'll write one if not.

A proper one, that would not only check that syscall didn't fail, but would also exercise the backlog, would require some time. I can't promise one soon. Feel free to write & commit yours. If I start working on a test, I will message you, so we don't do same work.