internal/poll: don't call SetFilePointerEx in Seek for overlapped handles

internal/poll: don't call SetFilePointerEx in Seek for overlapped handles

Overlapped handles don't have the file pointer updated when performing
I/O operations, so there is no need to call syscall.SetFilePointerEx in
FD.Seek. Updating the in-memory offset is sufficient.

Updates #74951 (provides a more complete fix)

Change-Id: Ibede6625cdbd501fc92cfdf8ce2782ec291af2b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/698035
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Damien Neil <dneil@google.com>