mld: convert ipv6_mc_socklist->sflist to RCU

The sflist has been protected by rwlock so that the critical section
is atomic context.
In order to switch this context, changing locking is needed.
The sflist actually already protected by RTNL So if it's converted
to use RCU, its control path context can be switched to sleepable.

Suggested-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>