ipv6: Fix route replacement with dev-only route

[ Upstream commit e404b8c7cfb31654c9024d497cec58a501501692 ]

After commit 2759647247 ("ipv6: fix ECMP route replacement") it is no
longer possible to replace an ECMP-able route by a non ECMP-able route.
For example,
	ip route add 2001:db8::1/128 via fe80::1 dev dummy0
	ip route replace 2001:db8::1/128 dev dummy0
does not work as expected.

Tweak the replacement logic so that point 3 in the log of the above commit
becomes:
3. If the new route is not ECMP-able, and no matching non-ECMP-able route
exists, replace matching ECMP-able route (if any) or add the new route.

We can now summarize the entire replace semantics to:
When doing a replace, prefer replacing a matching route of the same
"ECMP-able-ness" as the replace argument. If there is no such candidate,
fallback to the first route found.

Fixes: 2759647247 ("ipv6: fix ECMP route replacement")
Signed-off-by: Benjamin Poirier <bpoirier@cumulusnetworks.com>
Reviewed-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

net/ipv6/ip6_fib.c

@@ -893,8 +893,7 @@ static int fib6_add_rt2node(struct fib6_node *fn, struct rt6_info *rt,
 					found++;
 					break;
 				}
-				if (rt_can_ecmp)
-					fallback_ins = fallback_ins ?: ins;
+				fallback_ins = fallback_ins ?: ins;
 				goto next_iter;
 			}
 
@@ -934,7 +933,9 @@ next_iter:
 	}
 
 	if (fallback_ins && !found) {
-		/* No ECMP-able route found, replace first non-ECMP one */
+		/* No matching route with same ecmp-able-ness found, replace
+		 * first matching route
+		 */
 		ins = fallback_ins;
 		iter = *ins;
 		found++;