ip/ip6_gre: Fix non-point-to-point tunnel not generating IPv6 link local address

We recently found that our non-point-to-point tunnels were not
generating any IPv6 link local address and instead generating an
IPv6 compat address, breaking IPv6 communication on the tunnel.

Previously, addrconf_gre_config always would call addrconf_addr_gen
and generate a EUI64 link local address for the tunnel.
Then commit e5dd7294 changed the code path so that add_v4_addrs
is called but this only generates a compat IPv6 address for
non-point-to-point tunnels.

I assume the compat address is specifically for SIT tunnels so
have kept that only for SIT - GRE tunnels now always generate link
local addresses.

Fixes: e5dd7294 ("ip/ip6_gre: use the same logic as SIT interfaces when computing v6LL address")
Signed-off-by: Thomas Winter <Thomas.Winter@alliedtelesis.co.nz>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv6/addrconf.c

@@ -3127,17 +3127,17 @@ static void add_v4_addrs(struct inet6_dev *idev)
 		offset = sizeof(struct in6_addr) - 4;
 	memcpy(&addr.s6_addr32[3], idev->dev->dev_addr + offset, 4);
 
-	if (idev->dev->flags&IFF_POINTOPOINT) {
+	if (!(idev->dev->flags & IFF_POINTOPOINT) && idev->dev->type == ARPHRD_SIT) {
+		scope = IPV6_ADDR_COMPATv4;
+		plen = 96;
+		pflags |= RTF_NONEXTHOP;
+	} else {
 		if (idev->cnf.addr_gen_mode == IN6_ADDR_GEN_MODE_NONE)
 			return;
 
 		addr.s6_addr32[0] = htonl(0xfe800000);
 		scope = IFA_LINK;
 		plen = 64;
-	} else {
-		scope = IPV6_ADDR_COMPATv4;
-		plen = 96;
-		pflags |= RTF_NONEXTHOP;
 	}
 
 	if (addr.s6_addr32[3]) {