ice: fix broken Rx on VFs

Since the tagged commit, ice stopped respecting Rx buffer length
passed from VFs.
At that point, the buffer length was hardcoded in ice, so VFs still
worked up to some point (until, for example, a VF wanted an MTU
larger than its PF).
The next commit 93f53db ("ice: switch to Page Pool"), broke
Rx on VFs completely since ice started accounting per-queue buffer
lengths again, but now VF queues always had their length zeroed, as
ice was already ignoring what iavf was passing to it.

Restore the line that initializes the buffer length on VF queues
basing on the virtchnl messages.

Fixes: 3a4f419 ("ice: drop page splitting and recycling")
Reported-by: Jakub Slepecki <jakub.slepecki@intel.com>
Reviewed-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Tested-by: Jakub Slepecki <jakub.slepecki@intel.com>
Link: https://patch.msgid.link/20251124170735.3077425-1-aleksander.lobakin@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: alobakin

drivers/net/ethernet/intel/ice/virt/queues.c

@@ -842,6 +842,9 @@ int ice_vc_cfg_qs_msg(struct ice_vf *vf, u8 *msg)
 			    (qpi->rxq.databuffer_size > ((16 * 1024) - 128) ||
 			     qpi->rxq.databuffer_size < 1024))
 				goto error_param;
+
+			ring->rx_buf_len = qpi->rxq.databuffer_size;
+
 			if (qpi->rxq.max_pkt_size > max_frame_size ||
 			    qpi->rxq.max_pkt_size < 64)
 				goto error_param;