LDM: Fix for Windows Vista dynamic disks

This fixes the LDM driver so that it works with Windows Vista dynamic
disks which are subtly different to Windows 2000/XP ones.

The patch was needed to get a Vista formatted dynamic disk to be
recognized and parsed successfully.

Thanks go to Chris Teachworth for the report and testing.

Cc: Richard Russon <ldm@flatcap.org>
Signed-off-by: Anton Altaparmakov <aia21@cantab.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>