mld: respect RCU rules in ip6_mc_source() and ip6_mc_msfilter()

Whenever RCU protected list replaces an object,
the pointer to the new object needs to be updated
_before_ the call to kfree_rcu() or call_rcu()

Also ip6_mc_msfilter() needs to update the pointer
before releasing the mc_lock mutex.

Note that linux-5.13 was supporting kfree_rcu(NULL, rcu),
so this fix does not need the conditional test I was
forced to use in the equivalent patch for IPv4.

Fixes: 882ba1f7

 ("mld: convert ipv6_mc_socklist->sflist to RCU")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>