[NET]: Avoid copying TCP packets unnecessarily

[NET]: Avoid copying TCP packets unnecessarily

TCP packets all have writable heads, that is, even though it's cloned, it is
writable up to the end of the TCP header.  This patch makes skb_checksum_help
aware of this fact by using skb_clone_writable and avoiding a copy for TCP.

I've also modified the BUG_ON tests to be unsigned.  The only case where this
makes a difference is if csum_start points to a location before skb->data.
Since skb->data should always include the header where the checksum field
is (and all currently callers adhere to that), this change is safe and may
uncover bugs later.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>