internal/poll: advance file position in windows sendfile

Some versions of Windows (Windows 10 1803) do not set file
position after TransmitFile completes. So just use Seek
to set file position before returning from sendfile.

Fixes #25722

Change-Id: I7a49be10304b5db19dda707b13ac93d338aeb190
Reviewed-on: https://go-review.googlesource.com/131976


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Yasuhiro MATSUMOTO <mattn.jp@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>