tcp_nv: fix division by zero in tcpnv_acked()

tcp_nv: fix division by zero in tcpnv_acked()

BugLink: http://bugs.launchpad.net/bugs/1744213
[ Upstream commit 4eebff27ca4182bbf5f039dd60d79e2d7c0a707e ]

Average RTT could become zero. This happened in real life at least twice.
This patch treats zero as 1us.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Acked-by: Lawrence Brakmo <Brakmo@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>