fs/adfs: use format_version from disc_record

We only use the format version in one place during filesystem mount, so
it is pointless storing it in the superblock structure.  Also, we should
be using the version from the disc record in the map rather than the
boot block.

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