[TCP]: htcp - use measured rtt

Change HTCP to use measured RTT rather than smooth RTT.
Srtt is computed using the TCP receive timestamp
options, so it is vulnerable to hostile receivers. To avoid any problems
this might cause use the measured RTT instead.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>