net: fix fd leak with interfaces on aix/ppc64

To retrieve MTU on aix/ppc64, a socket must be created. Previously, this
socket was recreated for each interface and not close at all, causing a
fd leak on software using interface API.

Change-Id: Ib573e234bfce58964935831b68d007bfbd923476
Reviewed-on: https://go-review.googlesource.com/c/go/+/165397
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/interface_aix.go

@@ -5,6 +5,7 @@
 package net
 
 import (
+	"internal/poll"
 	"internal/syscall/unix"
 	"syscall"
 	"unsafe"
@@ -54,6 +55,12 @@ func interfaceTable(ifindex int) ([]Interface, error) {
 		return nil, err
 	}
 
+	sock, err := sysSocket(syscall.AF_INET, syscall.SOCK_DGRAM, 0)
+	if err != nil {
+		return nil, err
+	}
+	defer poll.CloseFunc(sock)
+
 	var ift []Interface
 	for len(tab) > 0 {
 		ifm := (*syscall.IfMsgHdr)(unsafe.Pointer(&tab[0]))
@@ -71,10 +78,6 @@ func interfaceTable(ifindex int) ([]Interface, error) {
 				// Retrieve MTU
 				ifr := &ifreq{}
 				copy(ifr.Name[:], ifi.Name)
-				sock, err := syscall.Socket(syscall.AF_INET, syscall.SOCK_DGRAM, 0)
-				if err != nil {
-					return nil, err
-				}
 				err = unix.Ioctl(sock, syscall.SIOCGIFMTU, uintptr(unsafe.Pointer(ifr)))
 				if err != nil {
 					return nil, err