[TCP] H-TCP: Use msecs_to_jiffies

Use functions to calculate jiffies from milliseconds and not the old,
crude method of dividing HZ by a value. Ensures more accurate values
even in the face of strange HZ values.

Signed-off-By: Baruch Even <baruch@ev-en.org>
Signed-off-by: David S. Miller <davem@davemloft.net>