net: skb_gro_checksum_* functions

Add skb_gro_checksum_validate, skb_gro_checksum_validate_zero_check,
and skb_gro_checksum_simple_validate, and __skb_gro_checksum_complete.
These are the cognates of the normal checksum functions but are used
in the gro_receive path and operate on GRO related fields in sk_buffs.

Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>