Kernel: Make sure IPv4Socket is marked as listening in listen()

Author: https://github.com/deoxxa
Commit: e8a10848b5
Pull-request: https://github.com/SerenityOS/serenity/pull/530
Reviewed-by: https://github.com/awesomekling

Kernel/Net/IPv4Socket.cpp

@@ -106,6 +106,7 @@ KResult IPv4Socket::listen(int backlog)
         return KResult(-EADDRINUSE);
 
     set_backlog(backlog);
+    m_role = Role::Listener;
 
     kprintf("IPv4Socket{%p} listening with backlog=%d\n", this, backlog);