net: add gro_compute_pseudo functions

Add inet_gro_compute_pseudo and ip6_gro_compute_pseudo. These are
the logical equivalents of inet_compute_pseudo and ip6_compute_pseudo
for GRO path. The IP header is taken from skb_gro_network_header.

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