Now that all BSDs define MNT_NOWAIT, use it in TestGetfsstat instead of
the locally defined copy.

Change-Id: Ib6cee08a6d3a034464c7b79591605b49889a46ae
Reviewed-on: https://go-review.googlesource.com/137195
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>