windows: implement Ftruncate using a single syscall on Windows

Ftruncate can be implemented on Windows using a single syscall. This
makes the implementation more efficient and less prone to races when
used in combination with other Seek calls.

Note that this is the x/sys counterpart for CL 618835.

Change-Id: Ie9be356bd953ccce85c0dd87a5dcc6ccf4fec464
Reviewed-on: https://go-review.googlesource.com/c/sys/+/621935
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Auto-Submit: Quim Muntal <quimmuntal@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Damien Neil <dneil@google.com>

windows/syscall_windows.go

@@ -725,20 +725,12 @@ func DurationSinceBoot() time.Duration {
 }
 
 func Ftruncate(fd Handle, length int64) (err error) {
-	curoffset, e := Seek(fd, 0, 1)
-	if e != nil {
-		return e
-	}
-	defer Seek(fd, curoffset, 0)
-	_, e = Seek(fd, length, 0)
-	if e != nil {
-		return e
+	type _FILE_END_OF_FILE_INFO struct {
+		EndOfFile int64
 	}
-	e = SetEndOfFile(fd)
-	if e != nil {
-		return e
-	}
-	return nil
+	var info _FILE_END_OF_FILE_INFO
+	info.EndOfFile = length
+	return SetFileInformationByHandle(fd, FileEndOfFileInfo, (*byte)(unsafe.Pointer(&info)), uint32(unsafe.Sizeof(info)))
 }
 
 func Gettimeofday(tv *Timeval) (err error) {