os: seek should invalidate any cached directory reads

When we seek on the underlying FD, discard any directory entries
we've already read and cached. This makes sure we won't return
the same entry twice.

We already fixed this for Darwin in CL 209961.

Fixes #37161

Change-Id: I20e1ac8d751443135e67fb4c43c18d69befb643b
Reviewed-on: https://go-review.googlesource.com/c/go/+/219143


Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>