net: ethtool: add get_rx_ring_count callback to optimize RX ring queries

net: ethtool: add get_rx_ring_count callback to optimize RX ring queries

Add a new optional get_rx_ring_count callback in ethtool_ops to allow
drivers to provide the number of RX rings directly without going through
the full get_rxnfc flow classification interface.

Create ethtool_get_rx_ring_count() to use .get_rx_ring_count if
available, falling back to get_rxnfc() otherwise. It needs to be
non-static, given it will be called by other ethtool functions laters,
as those calling get_rxfh().

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20250917-gxrings-v4-4-dae520e2e1cb@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>