batman-adv: upgrade package to latest release 2018.3

batman-adv: upgrade package to latest release 2018.3

* support latest kernels (3.16 - 4.19)
* coding style cleanups and refactoring
* enable the DAT by default for the in-tree Linux module
* bugs squashed:

  - fix segfault when writing to sysfs files under batman_adv/ with
    CONFIG_BATMAN_ADV_DEBUG enabled
  - avoid information leakage in probe ELP packets
  - adjust reference counters when queue_work fails
  - prevent duplicated entries in various lists

Signed-off-by: Sven Eckelmann <sven@narfation.org>