virtio-net: xsk: rx: move the xdp->data adjustment to buf_to_xdp()

This commit does not do any functional changes. It moves xdp->data
adjustment for buffer other than first buffer to buf_to_xdp() helper so
that the xdp_buff adjustment does not scatter over different functions.

Signed-off-by: Bui Quang Minh <minhquangbui99@gmail.com>
Link: https://patch.msgid.link/20250705075515.34260-1-minhquangbui99@gmail.com


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