On some systems removing files can cause a directory to be re-shuffled,
so simply continuing to read files can cause us to miss some.
Close and re-open the directory when looping, to avoid that.

Read more files each time through the loop, to reduce the chance of
having to re-open.

Fixes #20841

Change-Id: I98a14774ca63786ad05ba5000cbdb01ad2884332
Reviewed-on: https://go-review.googlesource.com/121255
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>