When something, on receive, between a CHECKSUM_HW using
driver and the code validating skb->csum trims the head
of the packet we have to reset skb->ip_summed back to
CHECKSUM_NONE because this kind of change invalidates
the skb->csum value calculated by the driver.

Various spots handle this properly, but not all.  So
abstract these actions into helper routines in order
to avoid code duplication.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>