net/mlx5: Extend Ethtool loopback selftest to support non-linear SKB

Current loopback test validation ignores non-linear SKB case in
the SKB access, which can lead to failures in scenarios such as
when HW GRO is enabled.
Linearize the SKB so both cases will be handled.

Signed-off-by: Alexei Lazar <alazar@nvidia.com>
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://patch.msgid.link/20250209101716.112774-15-tariqt@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>