net/mlx5: DR, Fix STEv1 incorrect L3 decapsulation padding

Decapsulation L3 on small inner packets which are less than
64 Bytes was done incorrectly. In small packets there is an
extra padding added in L2 which should not be included in L3
length. The issue was that after decapL3 the extra L2 padding
caused an update on the L3 length.

To avoid this issue the new header is pushed to the beginning
of the packet (offset 0) which should not cause a HW reparse
and update the L3 length.

Fixes: c349b413

 ("net/mlx5: DR, Add STEv1 modify header logic")
Reviewed-by: Erez Shitrit <erezsh@nvidia.com>
Reviewed-by: Yevgeny Kliteynik <kliteyn@nvidia.com>
Signed-off-by: Alex Vesker <valex@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>