net: update doc for unimplemented feature on Plan 9

Also removes unnecessary allocation.

Change-Id: I3406cf75a7b64d93b2b99c7f1f5c78f580452b60
Reviewed-on: https://go-review.googlesource.com/30891
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/interface.go

@@ -13,8 +13,8 @@ import (
 // BUG(mikio): On NaCl, methods and functions related to
 // Interface are not implemented.
 
-// BUG(mikio): On DragonFly BSD, NetBSD, OpenBSD and Solaris, the
-// MulticastAddrs method of Interface is not implemented.
+// BUG(mikio): On DragonFly BSD, NetBSD, OpenBSD, Plan 9 and Solaris,
+// the MulticastAddrs method of Interface is not implemented.
 
 var (
 	errInvalidInterface         = errors.New("invalid network interface")

src/net/interface_plan9.go

@@ -132,7 +132,7 @@ func interfaceCount() (int, error) {
 // network interfaces. Otherwise it returns addresses for a specific
 // interface.
 func interfaceAddrTable(ifi *Interface) ([]Addr, error) {
-	ifcs := []Interface{}
+	var ifcs []Interface
 	if ifi == nil {
 		var err error
 		ifcs, err = interfaceTable(0)
@@ -177,8 +177,7 @@ func interfaceAddrTable(ifi *Interface) ([]Addr, error) {
 		if !ok {
 			return nil, errors.New("cannot parse network mask for interface: " + status)
 		}
-		mask := CIDRMask(int(pfxlen), 128)
-
+		var mask IPMask
 		if ip.To4() != nil { // IPv4 or IPv6 IPv4-mapped address
 			mask = CIDRMask(pfxlen-8*len(v4InV6Prefix), 8*IPv4len)
 		}