internal/poll: pass the I/O mode instead of an overlapped object in execIO

internal/poll: pass the I/O mode instead of an overlapped object in execIO

execIO callers should be agnostic to the fact that it uses an overlapped
object. This will unlock future optimizations and simplifications.

Change-Id: I0a58d992101fa74ac75e3538af04cbc44156f0d6
Reviewed-on: https://go-review.googlesource.com/c/go/+/704175
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>