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>

drivers/staging/batman-adv/TODO

+ * Rework usage of RCU
+    - don't leak pointers from rcu out of rcu critical area which may
+      get freed
+    - check were synchronize_rcu must be used
+    - go through Documentation/RCU/checklist.txt
  * Request a new review
  * Process the comments from the review
  * Move into mainline proper