i40e: xsk: Move tmp desc array from driver to pool

Move desc_array from the driver to the pool. The reason behind this is
that we can then reuse this array as a temporary storage for descriptors
in all zero-copy drivers that use the batched interface. This will make
it easier to add batching to more drivers.

i40e is the only driver that has a batched Tx zero-copy
implementation, so no need to touch any other driver.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Alexander Lobakin <alexandr.lobakin@intel.com>
Link: https://lore.kernel.org/bpf/20220125160446.78976-6-maciej.fijalkowski@intel.com

include/net/xsk_buff_pool.h

@@ -60,6 +60,7 @@ struct xsk_buff_pool {
 	 */
 	dma_addr_t *dma_pages;
 	struct xdp_buff_xsk *heads;
+	struct xdp_desc *tx_descs;
 	u64 chunk_mask;
 	u64 addrs_cnt;
 	u32 free_list_cnt;