batman-adv: Fix netlink dumping of all mcast_flags buckets

The bucket variable is only updated outside the loop over the mcast_flags
buckets. It will only be updated during a dumping run when the dumping has
to be interrupted and a new message has to be started.

This could result in repeated or missing entries when the multicast flags
are dumped to userspace.

Fixes: d2d489b7 ("batman-adv: Add inconsistent multicast netlink dump detection")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>