net: core: let skb_partial_csum_set() set transport header

For untrusted packets with partial checksum, we need to set the transport header
for precise packet length estimation. We can just let skb_pratial_csum_set() to
do this to avoid extra call to skb_flow_dissect() and simplify the caller.

Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -3370,6 +3370,7 @@ bool skb_partial_csum_set(struct sk_buff *skb, u16 start, u16 off)
 	skb->ip_summed = CHECKSUM_PARTIAL;
 	skb->csum_start = skb_headroom(skb) + start;
 	skb->csum_offset = off;
+	skb_set_transport_header(skb, start);
 	return true;
 }
 EXPORT_SYMBOL_GPL(skb_partial_csum_set);