os: reject OpenDir of a non-directory file in Plan 9
Check that the path argument to OpenDir in Plan 9 is a directory,
and return error syscall.ENOTDIR if it is not.
Fixes #75196
Change-Id: I3bec6b6b40a38c21264b5d22ff3e7dfbf8c1c6d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/700855 Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: David du Colombier <0intro@gmail.com>