os: increase directory reading block size on Unix systems
os: increase directory reading block size on Unix systems

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 <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

nawawi/go,CAFxX/go,josharian/go,golang/go,nawawi/go,golang/go,josharian/go,akutz/go,cloudflare/go,josharian/go,nawawi/go,akutz/go,nawawi/go,josharian/go,akutz/go,nawawi/go,josharian/go,golang/go,CAFxX/go,golang/go,cloudflare/go,CAFxX/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,akutz/go,cloudflare/go,cloudflare/go,nawawi/go,josharian/go,CAFxX/go,josharian/go,nawawi/go,akutz/go,cloudflare/go,akutz/go,golang/go,cloudflare/go,akutz/go,cloudflare/go,golang/go,CAFxX/go,golang/go,josharian/go,akutz/go