xsk: Get rid of xdp_buff_xsk::xskb_list_node

Let's bring xdp_buff_xsk back to occupying 2 cachelines by removing
xskb_list_node - for the purpose of gathering the xskb frags
free_list_node can be used, head of the list (xsk_buff_pool::xskb_list)
stays as-is, just reuse the node ptr.

It is safe to do as a single xdp_buff_xsk can never reside in two
pool's lists simultaneously.

Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/bpf/20241007122458.282590-2-maciej.fijalkowski@intel.com