ice: Micro-optimize .ndo_xdp_xmit() path

After the recent mbuf changes, ice_xmit_xdp_ring() became a 3-liner.
It makes no sense to keep it global in a different file than its caller.
Move it just next to the sole call site and mark static. Also, it
doesn't need a full xdp_convert_frame_to_buff(). Save several cycles
and fill only the fields used by __ice_xmit_xdp_ring() later on.
Finally, since it doesn't modify @xdpf anyhow, mark the argument const
to save some more (whole -11 bytes of .text! :D).

Thanks to 1 jump less and less calcs as well, this yields as many as
6.7 Mpps per queue. `xdp.data_hard_start = xdpf` is fully intentional
again (see xdp_convert_buff_to_frame()) and just works when there are
no source device's driver issues.

Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/bpf/20230210170618.1973430-7-alexandr.lobakin@intel.com