vsock/vmci: fix sk_ack_backlog leak on failed handshake

When vmci_transport_recv_connecting_server() returns an error,
vmci_transport_recv_listen() calls vsock_remove_pending() but never
calls sk_acceptq_removed(). This leaves sk_ack_backlog incremented
permanently.

Repeated handshake failures (malformed packets, queue pair alloc
failure, event subscribe failure) cause sk_ack_backlog to climb
toward sk_max_ack_backlog. Once it reaches the limit the listener
permanently refuses all new connections with -ECONNREFUSED, a
silent denial of service requiring a process restart to recover.

The two existing sk_acceptq_removed() calls in af_vsock.c do not
cover this path: line 764 checks vsock_is_pending() which returns
false after vsock_remove_pending(), and line 1889 is only reached
on successful accept().

Fix by balancing sk_acceptq_added() with sk_acceptq_removed() on
the error path.

Fixes: d021c34405 ("VSOCK: Introduce VM Sockets")
Cc: stable@vger.kernel.org
Signed-off-by: Raf Dickson <rafdog35@gmail.com>
Acked-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://patch.msgid.link/20260526104356.469928-1-rafdog35@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/vmw_vsock/vmci_transport.c

@@ -980,8 +980,10 @@ static int vmci_transport_recv_listen(struct sock *sk,
 			err = -EINVAL;
 		}
 
-		if (err < 0)
+		if (err < 0) {
 			vsock_remove_pending(sk, pending);
+			sk_acceptq_removed(sk);
+		}
 
 		release_sock(pending);
 		vmci_transport_release_pending(pending);