internal/poll: use sync.Once instead to guard CopyFileRange() with kernel 5.3
The existing implementation creates more branches with more states: -1, 0, 1,
which makes it not very intuitive to understand, let's use sync.Once and boolean
instead to make it more straightforward.
Change-Id: I05766e5fdf7dba37d6565f84d3db4373f9342fe5
Reviewed-on: https://go-review.googlesource.com/c/go/+/425880
Run-TryBot: Ian Lance Taylor <iant@google.com> Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>