ipv6: fix net.ipv6.conf.all.accept_dad behaviour for real

Commit 35e015e1 ("ipv6: fix net.ipv6.conf.all interface DAD handlers")
was intended to affect accept_dad flag handling in such a way that
DAD operation and mode on a given interface would be selected
according to the maximum value of conf/{all,interface}/accept_dad.

However, addrconf_dad_begin() checks for particular cases in which we
need to skip DAD, and this check was modified in the wrong way.

Namely, it was modified so that, if the accept_dad flag is 0 for the
given interface *or* for all interfaces, DAD would be skipped.

We have instead to skip DAD if accept_dad is 0 for the given interface
*and* for all interfaces.

Fixes: 35e015e1 ("ipv6: fix net.ipv6.conf.all interface DAD handlers")
Acked-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Matteo Croce <mcroce@redhat.com>
Reported-by: Erik Kline <ek@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -3820,8 +3820,8 @@ static void addrconf_dad_begin(struct inet6_ifaddr *ifp)
 		goto out;
 
 	if (dev->flags&(IFF_NOARP|IFF_LOOPBACK) ||
-	    dev_net(dev)->ipv6.devconf_all->accept_dad < 1 ||
-	    idev->cnf.accept_dad < 1 ||
+	    (dev_net(dev)->ipv6.devconf_all->accept_dad < 1 &&
+	     idev->cnf.accept_dad < 1) ||
 	    !(ifp->flags&IFA_F_TENTATIVE) ||
 	    ifp->flags & IFA_F_NODAD) {
 		bump_id = ifp->flags & IFA_F_TENTATIVE;