xsk: avoid overwriting skb fields for multi-buffer traffic

xsk: avoid overwriting skb fields for multi-buffer traffic

We are unnecessarily setting a bunch of skb fields per each processed
descriptor, which is redundant for fragmented frames.

Let us set these respective members for first fragment only. To address
both paths that we have within xsk_build_skb(), move assignments onto
xsk_set_destructor_arg() and rename it to xsk_skb_init_misc().

Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250925160009.2474816-2-maciej.fijalkowski@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>