mac80211: Ensure initializing private mc_list in prepare_multicast().

Fix kernel panic by NULL pointer dereference in the context of
ieee80211_ops->prepare_multicast().

This bug was introduced by commit 22bedad3c.. ("net: convert
multicast list to list_head").

Call __hw_addr_init() in ieee80211_alloc_hw() to initialize
list_head of private device multicast list, like we do in
bond_init().

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Reviewed-by: Jiri Pirko <jpirko@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mac80211/main.c

@@ -388,6 +388,9 @@ struct ieee80211_hw *ieee80211_alloc_hw(size_t priv_data_len,
 	local->uapsd_max_sp_len = IEEE80211_DEFAULT_MAX_SP_LEN;
 
 	INIT_LIST_HEAD(&local->interfaces);
+
+	__hw_addr_init(&local->mc_list);
+
 	mutex_init(&local->iflist_mtx);
 	mutex_init(&local->scan_mtx);