xsk: make xsk_buff_add_frag() really add the frag via __xdp_buff_add_frag()

xsk: make xsk_buff_add_frag() really add the frag via __xdp_buff_add_frag()

Currently, xsk_buff_add_frag() only adds the frag to pool's linked list,
not doing anything with the &xdp_buff. The drivers do that manually and
the logic is the same.
Make it really add an skb frag, just like xdp_buff_add_frag() does that,
and freeing frags on error if needed. This allows to remove repeating
code from i40e and ice and not add the same code again and again.

Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Link: https://patch.msgid.link/20241218174435.1445282-5-aleksander.lobakin@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>