net: fix protocol number for IPv6 test
net: fix protocol number for IPv6 test

The protocol number of ICMP for IPv6 is 58, not 1.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/6810093

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang