net/mlx5e: Allow reporting of checksum unnecessary

[ Upstream commit b856df28f9230a47669efbdd57896084caadb2b3 ]

Currently we practically never report checksum unnecessary, because
for all IP packets we take the checksum complete path.

Enable non-default runs with reprorting checksum unnecessary, using
an ethtool private flag. This can be useful for performance evals
and other explorations.

Required by downstream patch which fixes XDP checksum.

Fixes: 86994156 ("net/mlx5e: XDP fast RX drop bpf programs support")
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>