bridge: fix icmpv6 endian bug and other sparse warnings

Fix the warnings reported by sparse on recent bridge multicast
changes. Mostly just rcu annotation issues but in this case
sparse found a real bug! The ICMPv6 mld2 query mrc
values is in network byte order.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>