i40e, xsk: Migrate to new MEM_TYPE_XSK_BUFF_POOL

Remove MEM_TYPE_ZERO_COPY in favor of the new MEM_TYPE_XSK_BUFF_POOL
APIs. The AF_XDP zero-copy rx_bi ring is now simply a struct xdp_buff
pointer.

v4->v5: Fixed "warning: Excess function parameter 'bi' description in
        'i40e_construct_skb_zc'". (Jakub)

Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Cc: intel-wired-lan@lists.osuosl.org
Link: https://lore.kernel.org/bpf/20200520192103.355233-9-bjorn.topel@gmail.com