Move eth_p_mpls() into mpls.h
Refactor skb_gso_segment MPLS handling into rpl_skb_gso_segment

Address Jesse's comments regarding this code:

"Can we push this completely into the skb_gso_segment() compatibility
code? It's both nicer and may make the interactions with the vlan code
less confusing."

netdevice.c would now like to reference ovs_skb_get_inner_protocol(),
but this is declared in datapath.h. Awaiting further direction to
resolve this dependency.
Move GSO compatibility code into linux/compat/gso.*
Refactor skb_network_protocol()