Given that skb_shared_info relies on skb_frag_t, in order to support
xskb chaining, introduce xdp_buff_xsk::xskb_list_node and
xsk_buff_pool::xskb_list.

This is needed so ZC drivers can add frags as xskb nodes which will make
it possible to handle it both when producing AF_XDP Rx descriptors as
well as freeing/recycling all the frags that a single frame carries.

Speaking of latter, update xsk_buff_free() to take care of list nodes.
For the former (adding as frags), introduce xsk_buff_add_frag() for ZC
drivers usage that is going to be used to add a frag to xskb list from
pool.

xsk_buff_get_frag() will be utilized by XDP_TX and, on contrary, will
return xdp_buff.

One of the previous patches added a wrapper for ZC Rx so implement xskb
list walk and production of Rx descriptors there.

On bind() path, bail out if socket wants to use ZC multi-buffer but
underlying netdev does not support it.
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/r/20230719132421.584801-12-maciej.fijalkowski@intel.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>