i40e: xsk: add RX multi-buffer support

i40e: xsk: add RX multi-buffer support

This patch is inspired from the multi-buffer support in non-zc path for
i40e as well as from the patch to support zc on ice. Each subsequent
frag is added to skb_shared_info of the first frag for possible xdp_prog
use as well to xsk buffer list for accessing the buffers in af_xdp.

For XDP_PASS, new pages are allocated for frags and contents are copied
from memory backed by xsk_buff_pool.

Replace next_to_clean with next_to_process as done in non-zc path and
advance it for every buffer and change the semantics of next_to_clean to
point to the first buffer of a packet. Driver will use next_to_process
in the same way next_to_clean was used previously.

For the non multi-buffer case, next_to_process and next_to_clean will
always be the same since each packet consists of a single buffer.

Signed-off-by: Tirthendu Sarkar <tirthendu.sarkar@intel.com>
Link: https://lore.kernel.org/r/20230719132421.584801-14-maciej.fijalkowski@intel.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>