fs/adfs: dir: add common directory buffer release method

With the bhs pointer in place, we have no need for separate per-format
free() methods, since a generic version will do.  Provide a generic
implementation, remove the format specific implementations and the
method function pointer.

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