internal/poll: specify current file position when calling TransmitFile

Current SendFile implementation assumes that TransmitFile starts from
the current file position. But that appears not true for Windows 10
Version 1803.

TransmitFile documentation

https://msdn.microsoft.com/en-us/library/windows/desktop/ms740565(v=vs.85).aspx

suggests, "You can use the lpOverlapped parameter to specify a 64-bit
offset within the file at which to start the file data transfer by
setting the Offset and OffsetHigh member of the OVERLAPPED structure."

Do as it advises.

Fixes #25722

Change-Id: I241d3bf76d0d5590d4df27c6f922d637068232fb
Reviewed-on: https://go-review.googlesource.com/117816
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/internal/poll/sendfile_windows.go

@@ -25,6 +25,16 @@ func SendFile(fd *FD, src syscall.Handle, n int64) (int64, error) {
 	o := &fd.wop
 	o.qty = uint32(n)
 	o.handle = src
+
+	// TODO(brainman): skip calling syscall.Seek if OS allows it
+	curpos, err := syscall.Seek(o.handle, 0, 1)
+	if err != nil {
+		return 0, err
+	}
+
+	o.o.OffsetHigh = uint32(curpos)
+	o.o.Offset = uint32(curpos >> 32)
+
 	done, err := wsrv.ExecIO(o, func(o *operation) error {
 		return syscall.TransmitFile(o.fd.Sysfd, o.handle, o.qty, 0, &o.o, nil, syscall.TF_WRITE_BEHIND)
 	})