net/eth: Introduce EthL4HdrProto

igb, a new network device emulation, will need SCTP checksum offloading.
Currently eth_get_protocols() has a bool parameter for each protocol
currently it supports, but there will be a bit too many parameters if
we add yet another protocol.

Introduce an enum type, EthL4HdrProto to represent all L4 protocols
eth_get_protocols() support with one parameter.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>