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/131976Reviewed-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>