[release-branch.go1.20] Revert "internal/poll: drop redundant ENOSYS in CopyFileRange"

[release-branch.go1.20] Revert "internal/poll: drop redundant ENOSYS in CopyFileRange"

This reverts CL 428555.

Reason for revert: It appears that even a newer kernel can get
ENOSYS from copy_file_range.

For #58592
Fixes #58627

Change-Id: Ib8dd1be61544f54bf652a99dc0b449109f8f50ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/470316
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>