fs/adfs: dir: add common dir object initialisation

Initialise the dir object before we pass it down to the directory format
specific read handler.  This allows us to get rid of the initialisation
inside those handlers.

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