net/ipv6: Fix a test against 'ipv6_find_idev()' return value

'ipv6_find_idev()' returns NULL on error, not an error pointer.
Update the test accordingly and return -ENOBUFS, as already done in
'addrconf_add_dev()', if NULL is returned.

Fixes: ("ipv6: allow userspace to add IFA_F_OPTIMISTIC addresses")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -4736,8 +4736,8 @@ inet6_rtm_newaddr(struct sk_buff *skb, struct nlmsghdr *nlh,
 			 IFA_F_MCAUTOJOIN | IFA_F_OPTIMISTIC;
 
 	idev = ipv6_find_idev(dev);
-	if (IS_ERR(idev))
-		return PTR_ERR(idev);
+	if (!idev)
+		return -ENOBUFS;
 
 	if (!ipv6_allow_optimistic_dad(net, idev))
 		cfg.ifa_flags &= ~IFA_F_OPTIMISTIC;