sk_buff: introduce 'slow_gro' flags

The new flag tracks if any state field is set, so that
GRO requires 'unusual'/slow prepare steps.

Set such flag when a ct entry is attached to the skb,
and never clear it.

The new bit uses an existing hole into the sk_buff struct

RFC -> v1:
 - use a single state bit, never clear it
 - avoid moving the _nfct field
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -689,6 +689,7 @@ typedef unsigned char *sk_buff_data_t;
  *		CHECKSUM_UNNECESSARY (max 3)
  *	@dst_pending_confirm: need to confirm neighbour
  *	@decrypted: Decrypted SKB
+ *	@slow_gro: state present at GRO time, slower prepare step required
  *	@napi_id: id of the NAPI struct this skb came from
  *	@sender_cpu: (aka @napi_id) source CPU in XPS
  *	@secmark: security marking
@@ -870,6 +871,7 @@ struct sk_buff {
 #ifdef CONFIG_TLS_DEVICE
 	__u8			decrypted:1;
 #endif
+	__u8			slow_gro:1;
 
 #ifdef CONFIG_NET_SCHED
 	__u16			tc_index;	/* traffic control index */
@@ -4216,6 +4218,7 @@ static inline unsigned long skb_get_nfct(const struct sk_buff *skb)
 static inline void skb_set_nfct(struct sk_buff *skb, unsigned long nfct)
 {
 #if IS_ENABLED(CONFIG_NF_CONNTRACK)
+	skb->slow_gro |= !!nfct;
 	skb->_nfct = nfct;
 #endif
 }
@@ -4375,6 +4378,7 @@ static inline void nf_copy(struct sk_buff *dst, const struct sk_buff *src)
 #if defined(CONFIG_NF_CONNTRACK) || defined(CONFIG_NF_CONNTRACK_MODULE)
 	nf_conntrack_put(skb_nfct(dst));
 #endif
+	dst->slow_gro = src->slow_gro;
 	__nf_copy(dst, src, true);
 }