vmxnet3: Fix missing reserved tailroom

vmxnet3: Fix missing reserved tailroom

Use rbi->len instead of rcd->len for non-dataring packet.

Found issue:
  XDP_WARN: xdp_update_frame_from_buff(line:278): Driver BUG: missing reserved tailroom
  WARNING: CPU: 0 PID: 0 at net/core/xdp.c:586 xdp_warn+0xf/0x20
  CPU: 0 PID: 0 Comm: swapper/0 Tainted: G        W  O       6.5.1 #1
  RIP: 0010:xdp_warn+0xf/0x20
  ...
  ? xdp_warn+0xf/0x20
  xdp_do_redirect+0x15f/0x1c0
  vmxnet3_run_xdp+0x17a/0x400 [vmxnet3]
  vmxnet3_process_xdp+0xe4/0x760 [vmxnet3]
  ? vmxnet3_tq_tx_complete.isra.0+0x21e/0x2c0 [vmxnet3]
  vmxnet3_rq_rx_complete+0x7ad/0x1120 [vmxnet3]
  vmxnet3_poll_rx_only+0x2d/0xa0 [vmxnet3]
  __napi_poll+0x20/0x180
  net_rx_action+0x177/0x390

Reported-by: Martin Zaharinov <micron10@gmail.com>
Tested-by: Martin Zaharinov <micron10@gmail.com>
Link: https://lore.kernel.org/netdev/74BF3CC8-2A3A-44FF-98C2-1E20F110A92E@gmail.com/
Fixes: 54f00cce1178 ("vmxnet3: Add XDP support.")
Signed-off-by: William Tu <witu@nvidia.com>
Link: https://lore.kernel.org/r/20240309183147.28222-1-witu@nvidia.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>