internall/poll: remove bufs field from Windows' poll.operation
The bufs field is used to avoid allocating it every time it is needed.
We can do better by using a sync.Pool to reuse allocations across
operations and FDs instead of the field.
A side benefit is that FD is now 16 bytes smaller and operation more
stateless.
Change-Id: I5d686d1526f6c63e7ca1ae84da1fbf2044b24703
Reviewed-on: https://go-review.googlesource.com/c/go/+/698798 Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Cherry Mui <cherryyz@google.com>