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/net/if_inet6.h b/include/net/if_inet6.h
index f95ff8d..04977ee 100644
--- a/include/net/if_inet6.h
+++ b/include/net/if_inet6.h
@@ -89,10 +89,11 @@
 struct ipv6_mc_socklist {
 	struct in6_addr		addr;
 	int			ifindex;
-	struct ipv6_mc_socklist *next;
+	struct ipv6_mc_socklist __rcu *next;
 	rwlock_t		sflock;
 	unsigned int		sfmode;		/* MCAST_{INCLUDE,EXCLUDE} */
 	struct ip6_sf_socklist	*sflist;
+	struct rcu_head		rcu;
 };
 
 struct ip6_sf_list {