batman-adv: Fix multicast TT issues with bogus ROAM flags

When a (broken) node wrongly sends multicast TT entries with a ROAM
flag then this causes any receiving node to drop all entries for the
same multicast MAC address announced by other nodes, leading to
packet loss.

Fix this DoS vector by only storing TT sync flags. For multicast TT
non-sync'ing flag bits like ROAM are unused so far anyway.

Fixes: 1d8ab8d3 ("batman-adv: Modified forwarding behaviour for multicast packets")
Reported-by: Leonardo Mörlein <me@irrelefant.net>
Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>