batman-adv: Avoid WARN_ON timing related checks

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

batman-adv: Avoid WARN_ON timing related checks

The soft/batadv interface for a queued OGM can be changed during the time

the OGM was queued for transmission and when the OGM is actually

transmitted by the worker.

But WARN_ON must be used to denote kernel bugs and not to print simple

warnings. A warning can simply be printed using pr_warn.