net: virtio_net: add get_rxrings ethtool callback for RX ring queries

net: virtio_net: add get_rxrings ethtool callback for RX ring queries

Replace the existing virtnet_get_rxnfc callback with a dedicated
virtnet_get_rxrings implementation to provide the number of RX rings
directly via the new ethtool_ops get_rx_ring_count pointer.

This simplifies the RX ring count retrieval and aligns virtio_net with
the new ethtool API for querying RX ring parameters.

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20250917-gxrings-v4-8-dae520e2e1cb@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>