net/mlx5: fix offset calculation for modify field action

net/mlx5: fix offset calculation for modify field action

Offsets are not taken into account during MAC addresses
manipulation for the MODIFY_FIELD action. That leads to
a wrong split between 0-15 and 16-47 bits and corrupted
data being copied to/from MAC addresses. Use both source
and destination offsets to calcucate the proper modify
header action specification.

Fixes: fdd0c046f4 ("net/mlx5: fix modify field action order for MAC")
Cc: stable@dpdk.org
Signed-off-by: Alexander Kozyrev <akozyrev@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>