os: handle 'no parent' error as IsNotExist on Plan 9

This error is returned by lib9p when removing a file
without parent. It should fix TestRemoveAllRace
when running on ramfs.

LGTM=bradfitz, aram
R=rsc, bradfitz, aram
CC=golang-codereviews, mischief
https://codereview.appspot.com/153410044