Teach LDM about a new field encountered with Windows Vista.

This fixes LDM for people using Vista who have disabled drive letter
assignment from one or more volumes.  Doing this introduces a so far
unknown field in the LDM database in the VOL5 VBLK structure which
causes the LDM driver to fail to parse the VBLK structure and hence LDM
fails to parse the disk altogether.  This patch teaches the driver about
this field.

Thanks got to Ashton Mills <amills@iinet.com.au> for reporting the
problem and working with me on getting it fixed.  It is now working for
him.
Signed-off-by: Anton Altaparmakov <aia21@cantab.net>
CC: Richard Russon <ldm@flatcap.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>