net: fix LookupSRV ordering on plan 9

lookup_plan9.go's lookupSRV is using the wrong order for srv results. order should be weight, priority, port, following the response from /net/dns:

  chi Aug  9 20:31:13 Rread tag 20 count 61 '_xmpp-client._tcp.offblast.org srv 5 0 5222 iota.offblast.org' 72

R=golang-dev, bradfitz
CC=ality, golang-dev, r, rsc
https://golang.org/cl/12708043

src/pkg/net/lookup_plan9.go

@@ -186,9 +186,9 @@ func lookupSRV(service, proto, name string) (cname string, addrs []*SRV, err err
 		if len(f) < 6 {
 			continue
 		}
-		port, _, portOk := dtoi(f[2], 0)
+		port, _, portOk := dtoi(f[4], 0)
 		priority, _, priorityOk := dtoi(f[3], 0)
-		weight, _, weightOk := dtoi(f[4], 0)
+		weight, _, weightOk := dtoi(f[2], 0)
 		if !(portOk && priorityOk && weightOk) {
 			continue
 		}