vvfat: introduce offset_to_bootsector, offset_to_fat and offset_to_root_dir

vvfat: introduce offset_to_bootsector, offset_to_fat and offset_to_root_dir

- offset_to_bootsector is the number of sectors up to FAT bootsector
- offset_to_fat is the number of sectors up to first File Allocation Table
- offset_to_root_dir is the number of sectors up to root directory sector

Replace first_sectors_number - 1 by offset_to_bootsector.
Replace first_sectors_number by offset_to_fat.
Replace faked_sectors by offset_to_rootdir.

Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>