fs/adfs: super: limit idlen according to directory type

Limit idlen according to the directory type, as idlen (the size of a
fragment ID) can not be more than 16 with the "new directory" layout.

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