dp-packet: Refactor offloading API.

dp-packet: Refactor offloading API.

1. No reason to have mbuf related APIs in a generic code.
2. Not only RSS/checksums should be invalidated in case of tunnel
   decapsulation or sending to 'ring' ports.

In order to fix two above issues, new function
'dp_packet_reset_offload' introduced. In order to clean up/unify
the code and simplify addition of new offloading features to non-DPDK
version of dp_packet, introduced 'ol_flags' bitmask. Additionally
reduced code complexity in 'dp_packet_clone_with_headroom' by using
already existent generic APIs.

Unfortunately, we still need to have a special case for mbuf
initialization inside 'dp_packet_init__()'.
'dp_packet_init_specific()' introduced for this purpose as a generic
API for initialization of the implementation-specific fields.

Acked-by: Flavio Leitner <fbl@sysclose.org>
Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Ian Stokes <ian.stokes@intel.com>