ipv6: mcast: RCU conversion

ipv6_sk_mc_lock rwlock becomes a spinlock.

readers (inet6_mc_check()) now takes rcu_read_lock() instead of read
lock. Writers dont need to disable BH anymore.

struct ipv6_mc_socklist objects are reclaimed after one RCU grace
period.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/ipv6.h b/include/linux/ipv6.h
index 8e429d0..0c99776 100644
--- a/include/linux/ipv6.h
+++ b/include/linux/ipv6.h
@@ -364,7 +364,7 @@
 
 	__u32			dst_cookie;
 
-	struct ipv6_mc_socklist	*ipv6_mc_list;
+	struct ipv6_mc_socklist	__rcu *ipv6_mc_list;
 	struct ipv6_ac_socklist	*ipv6_ac_list;
 	struct ipv6_fl_socklist *ipv6_fl_list;