gre: Fix IPv6 multicast route creation.

Use addrconf_add_dev() instead of ipv6_find_idev() in
addrconf_gre_config() so that we don't just get the inet6_dev, but also
install the default ff00::/8 multicast route.

Before commit 3e6a0243ff00 ("gre: Fix again IPv6 link-local address
generation."), the multicast route was created at the end of the
function by addrconf_add_mroute(). But this code path is now only taken
in one particular case (gre devices not bound to a local IP address and
in EUI64 mode). For all other cases, the function exits early and
addrconf_add_mroute() is not called anymore.

Using addrconf_add_dev() instead of ipv6_find_idev() in
addrconf_gre_config(), fixes the problem as it will create the default
multicast route for all gre devices. This also brings
addrconf_gre_config() a bit closer to the normal netdevice IPv6
configuration code (addrconf_dev_config()).

Cc: [e-mail]
Fixes: 3e6a0243ff00 ("gre: Fix again IPv6 link-local address generation.")
Reported-by: Aiden Yang <[e-mail]>
Closes: https://lore.kernel.org/netdev/CANR=[e-mail]/
Reviewed-by: Gary Guo <[e-mail]>
Tested-by: Gary Guo <[e-mail]>
Signed-off-by: Guillaume Nault <[e-mail]>
Reviewed-by: Ido Schimmel <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/ipv6/addrconf.c

···
 
 	ASSERT_RTNL();
 
-	idev = ipv6_find_idev(dev);
-	if (IS_ERR(idev)) {
-		pr_debug("%s: add_dev failed\n", __func__);
+	idev = addrconf_add_dev(dev);
+	if (IS_ERR(idev))
 		return;
-	}
 
 	/* Generate the IPv6 link-local address using addrconf_addr_gen(),
 	 * unless we have an IPv4 GRE device not bound to an IP address and
···
 	}
 
 	add_v4_addrs(idev);
-
-	if (dev->flags & IFF_POINTOPOINT)
-		addrconf_add_mroute(dev);
 }
 #endif