traceroute: properly reduce poll timeout

traceroute: properly reduce poll timeout

This removes the problem where during the time we wait to declare
a target as unresponsive we receive an unrelated ICMP packet.
If there is enough traffic, this can make traceroute hang as
it never declares the target as unresponsive.

function                                             old     new   delta
common_traceroute_main                              4196    4261     +65

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>