idpf: use generic functions to build xdp_buff and skb

idpf: use generic functions to build xdp_buff and skb

In preparation of XDP support, move from having skb as the main frame
container during the Rx polling to &xdp_buff.
This allows to use generic and libeth helpers for building an XDP
buffer and changes the logics: now we try to allocate an skb only
when we processed all the descriptors related to the frame.
Store &libeth_xdp_stash instead of the skb pointer on the Rx queue.
It's only 8 bytes wider, but contains everything we may need.

Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Tested-by: Ramu R <ramu.r@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>