net: Fix possible wrong checksum generation.

Patch cef401de7b (net: fix possible wrong checksum
generation) fixed wrong checksum calculation but it broke TSO by
defining new GSO type but not a netdev feature for that type.
net_gso_ok() would not allow hardware checksum/segmentation
offload of such packets without the feature.

Following patch fixes TSO and wrong checksum. This patch uses
same logic that Eric Dumazet used. Patch introduces new flag
SKBTX_SHARED_FRAG if at least one frag can be modified by
the user. but SKBTX_SHARED_FRAG flag is kept in skb shared
info tx_flags rather than gso_type.

tx_flags is better compared to gso_type since we can have skb with
shared frag without gso packet. It does not link SHARED_FRAG to
GSO, So there is no need to define netdev feature for this.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_output.c

@@ -598,6 +598,7 @@ slow_path:
 	/* for offloaded checksums cleanup checksum before fragmentation */
 	if ((skb->ip_summed == CHECKSUM_PARTIAL) && skb_checksum_help(skb))
 		goto fail;
+	iph = ip_hdr(skb);
 
 	left = skb->len - hlen;		/* Space per frame */
 	ptr = hlen;		/* Where to start from */