gro: Fix handling of imprecisely split packets

The commit 89a1b249edcf9be884e71f92df84d48355c576aa (gro: Avoid
copying headers of unmerged packets) only worked for packets
which are either completely linear, completely non-linear, or
packets which exactly split at the boundary between headers and
payload.

Anything else would cause bits in the header to go missing if
the packet is held by GRO.

This may have broken drivers such as ixgbe.

This patch fixes the places that assumed or only worked with
the above cases.

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