Staging: batman-adv: Add rcu TODO

Paul E. McKenney informed us that rcu is misused by leaking pointers to
rcu related elements outside read-side protected critical sections.

He also recommended that it should be checked against the rcu checklist.

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