net: relax expensive skb_unclone() in iptunnel_handle_offloads()

Locally generated TCP GSO packets having to go through a GRE/SIT/IPIP
tunnel have to go through an expensive skb_unclone()

Reallocating skb->head is a lot of work.

Test should really check if a 'real clone' of the packet was done.

TCP does not care if the original gso_type is changed while the packet
travels in the stack.

This adds skb_header_unclone() which is a variant of skb_clone()
using skb_header_cloned() check instead of skb_cloned().

This variant can probably be used from other points.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -1325,6 +1325,16 @@ static inline int skb_header_cloned(const struct sk_buff *skb)
 	return dataref != 1;
 }
 
+static inline int skb_header_unclone(struct sk_buff *skb, gfp_t pri)
+{
+	might_sleep_if(gfpflags_allow_blocking(pri));
+
+	if (skb_header_cloned(skb))
+		return pskb_expand_head(skb, 0, 0, pri);
+
+	return 0;
+}
+
 /**
  *	skb_header_release - release reference to header
  *	@skb: buffer to operate on

net/ipv4/ip_tunnel_core.c

@@ -157,7 +157,7 @@ int iptunnel_handle_offloads(struct sk_buff *skb,
 	}
 
 	if (skb_is_gso(skb)) {
-		err = skb_unclone(skb, GFP_ATOMIC);
+		err = skb_header_unclone(skb, GFP_ATOMIC);
 		if (unlikely(err))
 			return err;
 		skb_shinfo(skb)->gso_type |= gso_type_mask;