Reportedly CIFS on RHEL 7 can fail to report files if directories are
read in 4K increments. While this seems to be a CIFS or RHEL bug,
reportedly CIFS does not return more than 5760 bytes in a block, so
reading in 8K increments should hide the problem from users with
minimal cost.

Fixes #24015

Change-Id: Iaf9f00ffe338d379c819ed9edcd4cc9834e3b0f7
Reviewed-on: https://go-review.googlesource.com/121756
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>