batman-adv: Move common genl doit code pre/post hooks

The commit ff4c92d8 ("genetlink: introduce pre_doit/post_doit hooks")
intoduced a mechanism to run specific code for doit hooks before/after the
hooks are run. Since all doit hooks are requiring the batadv softif, it
should be retrieved/freed in these helpers to simplify the code.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>