internal/poll: check return value instead of errno for copy_file_range(2)
There is one special case of (0, nil) indicating EOF where the updates
of zero to remain and written are redundant.
Change-Id: I017471657a9424fab88c72d14d3eb66d14a7e5c7
Reviewed-on: https://go-review.googlesource.com/c/go/+/609297
Run-TryBot: Andy Pan <panjf2000@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>