VSOCK: Don't dec ack backlog twice for rejected connections

If a pending socket is marked as rejected, we will decrease the
sk_ack_backlog twice. So don't decrement it for rejected sockets
in vsock_pending_work().

Testing of the rejected socket path was done through code
modifications.

Reported-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Jorgen Hansen <jhansen@vmware.com>
Reviewed-by: Adit Ranadive <aditr@vmware.com>
Reviewed-by: Aditya Sarwade <asarwade@vmware.com>
Signed-off-by: David S. Miller <davem@davemloft.net>