net: Add net protocol offload registration infrustructure

Create a new data structure for IPv4 protocols that holds GRO/GSO
callbacks and a new array to track the protocols that register GRO/GSO.

Signed-off-by: Vlad Yasevich <vyasevic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>