batman-adv: Drop unmanaged ELP metric worker

In the Linux kernel, the following vulnerability has been resolved:

batman-adv: Drop unmanaged ELP metric worker

The ELP worker needs to calculate new metric values for all neighbors

"reachable" over an interface. Some of the used metric sources require

locks which might need to sleep. This sleep is incompatible with the RCU

list iterator used for the recorded neighbors. The initial approach to work

around of this problem was to queue another work item per neighbor and then

run this in a new context.

Even when this solved the RCU vs might_sleep() conflict, it has a major

problems: Nothing was stopping the work item in case it is not needed

anymore - for example because one of the related interfaces was removed or

the batman-adv module was unloaded - resulting in potential invalid memory

accesses.

Directly canceling the metric worker also has various problems:

* cancel_work_sync for a to-be-deactivated interface is called with

rtnl_lock held. But the code in the ELP metric worker also tries to use

rtnl_lock() - which will never return in this case. This also means that

cancel_work_sync would never return because it is waiting for the worker

to finish.

* iterating over the neighbor list for the to-be-deactivated interface is

currently done using the RCU specific methods. Which means that it is

possible to miss items when iterating over it without the associated

spinlock - a behaviour which is acceptable for a periodic metric check

but not for a cleanup routine (which must "stop" all still running

workers)

The better approch is to get rid of the per interface neighbor metric

worker and handle everything in the interface worker. The original problems

are solved by:

* creating a list of neighbors which require new metric information inside

the RCU protected context, gathering the metric according to the new list

outside the RCU protected context

* only use rcu_trylock inside metric gathering code to avoid a deadlock

when the cancel_delayed_work_sync is called in the interface removal code

(which is called with the rtnl_lock held)