os: fix error returned by dirstat on Plan 9

os: fix error returned by dirstat on Plan 9

When the Stat or Fstat system calls return -1,
dirstat incorrectly returns ErrShortStat.

However, the error returned by Stat or Fstat
could be different. For example, when the
file doesn't exist, they return "does not exist".

Dirstat should return the error returned by
the system call.

Fixes #10911.
Fixes #11132.

Change-Id: Icf242d203d256f12366b1e277f99b1458385104a
Reviewed-on: https://go-review.googlesource.com/10900
Run-TryBot: David du Colombier <0intro@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>