tcp: fix ICMP-RTO war

Make sure, that TCP has a nonzero RTT estimation after three-way
handshake. Currently, a listening TCP has a value of 0 for srtt,
rttvar and rto right after the three-way handshake is completed
with TCP timestamps disabled.
This will lead to corrupt RTO recalculation and retransmission
flood when RTO is recalculated on backoff reversion as introduced
in "Revert RTO on ICMP destination unreachable"
(f1ecd5d9).
This behaviour can be provoked by connecting to a server which
"responds first" (like SMTP) and rejecting every packet after
the handshake with dest-unreachable, which will lead to softirq
load on the server (up to 30% per socket in some tests).

Thanks to Ilpo Jarvinen for providing debug patches and to
Denys Fedoryshchenko for reporting and testing.

Changes since v3: Removed bad characters in patchfile.

Reported-by: Denys Fedoryshchenko <denys@visp.net.lb>
Signed-off-by: Damian Lukowski <damian@tvk.rwth-aachen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>