netfilter: Switch to skb_dstref_steal to clear dst_entry

Going forward skb_dst_set will assert that skb dst_entry
is empty during skb_dst_set. skb_dstref_steal is added to reset
existing entry without doing refcnt. Switch to skb_dstref_steal
in ip[6]_route_me_harder and add a comment on why it's safe
to skip skb_dstref_restore.

Acked-by: Florian Westphal <[e-mail]>
Signed-off-by: Stanislav Fomichev <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/ipv4/netfilter.c

···
 	if (!(IPCB(skb)->flags & IPSKB_XFRM_TRANSFORMED) &&
 	    xfrm_decode_session(net, skb, flowi4_to_flowi(&fl4), AF_INET) == 0) {
 		struct dst_entry *dst = skb_dst(skb);
-		skb_dst_set(skb, NULL);
+		/* ignore return value from skb_dstref_steal, xfrm_lookup takes
+		 * care of dropping the refcnt if needed.
+		 */
+		skb_dstref_steal(skb);
 		dst = xfrm_lookup(net, dst, flowi4_to_flowi(&fl4), sk, 0);
 		if (IS_ERR(dst))
 			return PTR_ERR(dst);

net/ipv6/netfilter.c

···
 #ifdef CONFIG_XFRM
 	if (!(IP6CB(skb)->flags & IP6SKB_XFRM_TRANSFORMED) &&
 	    xfrm_decode_session(net, skb, flowi6_to_flowi(&fl6), AF_INET6) == 0) {
-		skb_dst_set(skb, NULL);
+		/* ignore return value from skb_dstref_steal, xfrm_lookup takes
+		 * care of dropping the refcnt if needed.
+		 */
+		skb_dstref_steal(skb);
 		dst = xfrm_lookup(net, dst, flowi6_to_flowi(&fl6), sk, 0);
 		if (IS_ERR(dst))
 			return PTR_ERR(dst);