[IPV6]: Fix raw socket checksums with IPsec

I made a mistake in my last patch to the raw socket checksum code.
I used the value of inet->cork.length as the length of the payload.
While this works with normal packets, it breaks down when IPsec is
present since the cork length includes the extension header length.

So here is a patch to fix the length calculations.

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