batman-adv: Add debugfs table for mcast flags

This patch adds a debugfs table with originators and their according
multicast flags to help users figure out why multicast optimizations
might be enabled or disabled for them.

Tested-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>