internal/poll: if copy_file_range returns 0, assume it failed

On current Linux kernels copy_file_range does not correctly handle
files in certain special file systems, such as /proc. For those file
systems it fails to copy any data and returns zero. This breaks Go's
io.Copy for those files.

Fix the problem by assuming that if copy_file_range returns 0 the
first time it is called on a file, that that file is not supported.
In that case fall back to just using read. This will force an extra
system call when using io.Copy to copy a zero-sized normal file,
but at least it will work correctly.

For #36817
Fixes #44272

Change-Id: I02e81872cb70fda0ce5485e2ea712f219132e614
Reviewed-on: https://go-review.googlesource.com/c/go/+/291989


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>