net: ethtool: pass the num of RX rings directly to ethtool_copy_validate_indir

net: ethtool: pass the num of RX rings directly to ethtool_copy_validate_indir

Modify ethtool_copy_validate_indir() and callers to validate indirection
table entries against the number of RX rings as an integer instead of
accessing rx_rings->data.

This will be useful in the future, given that struct ethtool_rxnfc might
not exist for native GRXRINGS call.

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20250917-gxrings-v4-1-dae520e2e1cb@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>