net: ethtool: provide shims for stats aggregation helpers when CONFIG_ETHTOOL_NETLINK=n

net: ethtool: provide shims for stats aggregation helpers when CONFIG_ETHTOOL_NETLINK=n

ethtool_aggregate_*_stats() are implemented in net/ethtool/stats.c, a
file which is compiled out when CONFIG_ETHTOOL_NETLINK=n. In order to
avoid adding Kbuild dependencies from drivers (which call these helpers)
on CONFIG_ETHTOOL_NETLINK, let's add some shim definitions which simply
make the helpers dead code.

This means the function prototypes should have been located in
include/linux/ethtool_netlink.h rather than include/linux/ethtool.h.

Fixes: 449c5459641a ("net: ethtool: add helpers for aggregate statistics")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Link: https://lore.kernel.org/r/20230125110214.4127759-1-vladimir.oltean@nxp.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>