mbuf_offload: introduce library to attach offloads to mbuf

mbuf_offload: introduce library to attach offloads to mbuf

This library add support for adding a chain of offload operations to a
mbuf. It contains the definition of the rte_mbuf_offload structure as
well as helper functions for attaching  offloads to mbufs and a mempool
management functions.

This initial implementation supports attaching multiple offload
operations to a single mbuf, but only a single offload operation of a
specific type can be attach to that mbuf.

Signed-off-by: Declan Doherty <declan.doherty@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>