net/mlx5: convert meta register to big-endian

net/mlx5: convert meta register to big-endian

Metadata were stored in the CPU order (little-endian format on x86),
while all the packet header fields are stored in the network order.
That caused wrong results whenever we tried to use metadata value
in the modify_field action: bytes were swapped as a result.

Convert the metadata value into big-endian format before storing it
in the Mellanox NIC to achieve consistent behaviour.

Fixes: 641dbe4fb053 ("net/mlx5: support modify field flow action")
Cc: stable@dpdk.org
Signed-off-by: Alexander Kozyrev <akozyrev@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>