syscall: fix SockaddrDatalink on netbsd
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
http://codereview.appspot.com/6223051
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go