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rework mvneta napi_poll loop for XDP multi-buffers
Rework mvneta_rx_swbm routine in order to process all rx descriptors before
building the skb or run the xdp program attached to the interface.
Introduce xdp_get_shared_info_from_{buff,frame} utility routines to get the
skb_shared_info pointer from xdp_buff or xdp_frame.
This is a preliminary series to enable multi-buffers and jumbo frames for XDP
according to [1]