Staging: batman-adv: Add hash recommendations to TODO

David Miller <davem@davemloft.net> recommended in
20100924.134334.28812338.davem@davemloft.net that we must make the hash
abstraction helper more efficient and may remove it completely.

Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>