internal/poll: use sync.OnceValue to determine kernel version ≥ 5.3 in CopyFileRange

internal/poll: use sync.OnceValue to determine kernel version ≥ 5.3 in CopyFileRange

Change-Id: I13fdf86c3f46bf3c83cb116e9dd3bc4ab1a949d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/573755
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: qiulaidongfeng <2645477756@qq.com>