sk_buff: track dst status in slow_gro

Similar to the previous patch, but covering the dst field:
the slow_gro flag is additionally set when a dst is attached
to the skb

RFC -> v1:
 - use the existing flag instead of adding a new one
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -992,6 +992,7 @@ static inline struct dst_entry *skb_dst(const struct sk_buff *skb)
  */
 static inline void skb_dst_set(struct sk_buff *skb, struct dst_entry *dst)
 {
+	skb->slow_gro |= !!dst;
 	skb->_skb_refdst = (unsigned long)dst;
 }
 
@@ -1008,6 +1009,7 @@ static inline void skb_dst_set(struct sk_buff *skb, struct dst_entry *dst)
 static inline void skb_dst_set_noref(struct sk_buff *skb, struct dst_entry *dst)
 {
 	WARN_ON(!rcu_read_lock_held() && !rcu_read_lock_bh_held());
+	skb->slow_gro = !!dst;
 	skb->_skb_refdst = (unsigned long)dst | SKB_DST_NOREF;
 }
 

include/net/dst.h

@@ -277,6 +277,7 @@ static inline void skb_dst_drop(struct sk_buff *skb)
 
 static inline void __skb_dst_copy(struct sk_buff *nskb, unsigned long refdst)
 {
+	nskb->slow_gro |= !!refdst;
 	nskb->_skb_refdst = refdst;
 	if (!(nskb->_skb_refdst & SKB_DST_NOREF))
 		dst_clone(skb_dst(nskb));
@@ -316,6 +317,7 @@ static inline bool skb_dst_force(struct sk_buff *skb)
 			dst = NULL;
 
 		skb->_skb_refdst = (unsigned long)dst;
+		skb->slow_gro |= !!dst;
 	}
 
 	return skb->_skb_refdst != 0UL;