[release-branch.go1.15] internal/poll: treat copy_file_range EIO as not-handled

For #42334
Fixes #42369

Change-Id: Ife51df4e7d2539a04393abfdec45e3f902975fca
Reviewed-on: https://go-review.googlesource.com/c/go/+/266940


Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
(cherry picked from commit 633f9e20)
Reviewed-on: https://go-review.googlesource.com/c/go/+/267917


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>