batman-adv: multicast TT fixes and cleanups

The first one adds a fix that might potentially result in multicast packet
loss once we would enable multicast_mode again.

The second one avoids some small but unnecessary overhead. More
importantly though, it is supposed to ease further multicast improvements
later (e.g. no need for a multicast sending node to determine overlap
between WANT_ALL_IPV4/6 flags and TT entries while on fast-path).

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>