net: offload: More decoupling of net_offload from l2 naming

This is part of a patch series to decouple TCP/IP offload, which
currently occurs at the transport layer (L4), from the data link
layer (L2).

- Rename struct net_l2_offload_ip to struct net_offload
- Rename struct field offload_ip to just offload
- Rename include/net/offload_ip.h -> include/net/net_offload.h

Change-Id: I3cd891c2b13e0e8f3ad1c66264f90b5031ae17c2
Signed-off-by: Gil Pitney <gil.pitney@linaro.org>