syscall: fix marshaling of stat messages on Plan 9
syscall: fix marshaling of stat messages on Plan 9

The order of the Qid fields was reversed. Mea culpa.

R=seed
CC=golang-dev
https://codereview.appspot.com/7231062
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go