batman-adv: mcast: fix use-after-free in orig_node RCU release

batman-adv: mcast: fix use-after-free in orig_node RCU release

batadv_mcast_purge_orig() removes entries from RCU-protected hlists but
does not wait for an RCU grace period before returning. Concurrent RCU
readers may still accesses references to those entries at the point of
removal. RCU-protected readers trying to operate on entries like
orig->mcast_want_all_ipv6_node will then access already freed memory.

Fix this by moving batadv_mcast_purge_orig() to batadv_orig_node_release(),
just before the call_rcu() invocation. This ensures RCU readers that were
active at purge time have drained before the orig_node memory is reclaimed.

Cc: stable@kernel.org
Fixes: ab49886e3da7 ("batman-adv: Add IPv4 link-local/IPv6-ll-all-nodes multicast support")
Acked-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Sven Eckelmann <sven@narfation.org>