net: fix LookupNS on Plan 9
net: fix LookupNS on Plan 9

use correct field count when resolving nameservers via /net/dns on Plan 9.

we incorrectly check for 4 fields instead of 3 when parsing the result of /net/dns, and get no results

R=golang-dev, ality
CC=golang-dev
https://codereview.appspot.com/10182044

Committer: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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