xsk: wrap generic metadata handling onto separate function

xsk_build_skb() has gone wild with its size and one of the things we can
do about it is to pull out a branch that takes care of metadata handling
and make it a separate function.

While at it, let us add metadata SW support for devices supporting
IFF_TX_SKB_NO_LINEAR flag, that happen to have separate logic for
building skb in xsk's generic xmit path.

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250925160009.2474816-4-maciej.fijalkowski@intel.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>