gve: Fix RX_BUFFERS_POSTED stat to report per-queue fill_cnt

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 <alok.a.tiwari@oracle.com>
Link: https://patch.msgid.link/20250527130830.1812903-1-alok.a.tiwari@oracle.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>