net: vrf: Fix dst reference counting

Vivek reported a kernel exception deleting a VRF with an active
connection through it. The root cause is that the socket has a cached
reference to a dst that is destroyed. Converting the dst_destroy to
dst_release and letting proper reference counting kick in does not
work as the dst has a reference to the device which needs to be released
as well.

I talked to Hannes about this at netdev and he pointed out the ipv4 and
ipv6 dst handling has dst_ifdown for just this scenario. Rather than
continuing with the reinvented dst wheel in VRF just remove it and
leverage the ipv4 and ipv6 versions.

Fixes: 193125dbd8eb2 ("net: Introduce VRF device driver")
Fixes: 35402e3136634 ("net: Add IPv6 support to VRF device")

Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index ed44663..1d8871a 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -338,9 +338,9 @@
 	return rt;
 }
 
-static struct rt6_info *ip6_dst_alloc(struct net *net,
-				      struct net_device *dev,
-				      int flags)
+struct rt6_info *ip6_dst_alloc(struct net *net,
+			       struct net_device *dev,
+			       int flags)
 {
 	struct rt6_info *rt = __ip6_dst_alloc(net, dev, flags);
 
@@ -364,6 +364,7 @@
 
 	return rt;
 }
+EXPORT_SYMBOL(ip6_dst_alloc);
 
 static void ip6_dst_destroy(struct dst_entry *dst)
 {