batman-adv: Add log_level mesh genl configuration

In contrast to other modules, batman-adv allows to set the debug message
verbosity per mesh/soft-interface and not per module (via modparam).

The BATADV_CMD_SET_MESH/BATADV_CMD_GET_MESH commands allow to set/get the
configuration of this feature using the u32 (bitmask) BATADV_ATTR_LOG_LEVEL
attribute.

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