gve: Fix RX_BUFFERS_POSTED stat to report per-queue fill_cnt

Previously, the RX_BUFFERS_POSTED stat incorrectly reported the
fill_cnt from RX queue 0 for all queues, resulting in inaccurate
per-queue statistics.
Fix this by correctly indexing priv->rx[idx].fill_cnt for each RX queue.

Fixes: 24aeb56f2d38 ("gve: Add Gvnic stats AQ command and ethtool show/set-priv-flags.")
Signed-off-by: Alok Tiwari <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

drivers/net/ethernet/google/gve/gve_main.c

···
 			};
 			stats[stats_idx++] = (struct stats) {
 				.stat_name = cpu_to_be32(RX_BUFFERS_POSTED),
-				.value = cpu_to_be64(priv->rx[0].fill_cnt),
+				.value = cpu_to_be64(priv->rx[idx].fill_cnt),
 				.queue_id = cpu_to_be32(idx),
 			};
 		}