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: 882ba1f ("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>

net/ipv6/mcast.c

@@ -460,10 +460,10 @@ int ip6_mc_source(int add, int omode, struct sock *sk,
 				newpsl->sl_addr[i] = psl->sl_addr[i];
 			atomic_sub(struct_size(psl, sl_addr, psl->sl_max),
 				   &sk->sk_omem_alloc);
-			kfree_rcu(psl, rcu);
 		}
+		rcu_assign_pointer(pmc->sflist, newpsl);
+		kfree_rcu(psl, rcu);
 		psl = newpsl;
-		rcu_assign_pointer(pmc->sflist, psl);
 	}
 	rv = 1;	/* > 0 for insert logic below if sl_count is 0 */
 	for (i = 0; i < psl->sl_count; i++) {
@@ -565,12 +565,12 @@ int ip6_mc_msfilter(struct sock *sk, struct group_filter *gsf,
 			       psl->sl_count, psl->sl_addr, 0);
 		atomic_sub(struct_size(psl, sl_addr, psl->sl_max),
 			   &sk->sk_omem_alloc);
-		kfree_rcu(psl, rcu);
 	} else {
 		ip6_mc_del_src(idev, group, pmc->sfmode, 0, NULL, 0);
 	}
-	mutex_unlock(&idev->mc_lock);
 	rcu_assign_pointer(pmc->sflist, newpsl);
+	mutex_unlock(&idev->mc_lock);
+	kfree_rcu(psl, rcu);
 	pmc->sfmode = gsf->gf_fmode;
 	err = 0;
 done: