fs/adfs: map: fix map scanning

When scanning the map for a fragment id, we need to keep track of the
free space links, so we don't inadvertently believe that the freespace
link is a valid fragment id.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>