[TCP] Westwood: bandwidth filter startup

The bandwidth estimate filter is now initialized with the first
sample in order to have better performances in the case of small
file transfers.

Signed-off-by: Luca De Cicco <ldecicco@gmail.com>
Signed-off-by: Stephen Hemminger <shemminger@dxpl.pdx.osdl.net>
Signed-off-by: David S. Miller <davem@davemloft.net>