route: Use the device mtu as the default for blackhole routes

As it is, we return null as the default mtu of blackhole routes.
This may lead to a propagation of a bogus pmtu if the default_mtu
method of a blackhole route is invoked. So return dst->dev->mtu
as the default mtu instead.
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -2757,7 +2757,7 @@ static struct dst_entry *ipv4_blackhole_dst_check(struct dst_entry *dst, u32 coo
 
 static unsigned int ipv4_blackhole_default_mtu(const struct dst_entry *dst)
 {
-	return 0;
+	return dst->dev->mtu;
 }
 
 static void ipv4_rt_blackhole_update_pmtu(struct dst_entry *dst, u32 mtu)

net/ipv6/route.c

@@ -157,7 +157,7 @@ static struct dst_ops ip6_dst_ops_template = {
 
 static unsigned int ip6_blackhole_default_mtu(const struct dst_entry *dst)
 {
-	return 0;
+	return dst->dev->mtu;
 }
 
 static void ip6_rt_blackhole_update_pmtu(struct dst_entry *dst, u32 mtu)