tcp: convert cached rtt from usec to jiffies when feeding initial rto

tcp: convert cached rtt from usec to jiffies when feeding initial rto

BugLink: http://bugs.launchpad.net/bugs/1573034
[ Upstream commit 9bdfb3b79e61c60e1a3e2dc05ad164528afa6b8a ]

Currently it's converted into msecs, thus HZ=1000 intact.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Fixes: 740b0f1841f6 ("tcp: switch rtt estimations to usec resolution")
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>