syscall: fix SockaddrDatalink on netbsd

RawSockaddrDatalink and SockaddrDatalink need to match - make Data
have length 12 for both.

R=golang-dev, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/6223051

src/pkg/syscall/syscall_netbsd.go

@@ -22,7 +22,7 @@ type SockaddrDatalink struct {
 	Nlen   uint8
 	Alen   uint8
 	Slen   uint8
-	Data   [24]int8
+	Data   [12]int8
 	raw    RawSockaddrDatalink
 }