net/mlx5: DR, Fix postsend actions write length

Fix the send info write length to be (actions x action) size in bytes.

Fixes: 297ccceb

 ("net/mlx5: DR, Expose an internal API to issue RDMA operations")
Signed-off-by: Hamdan Igbaria <hamdani@mellanox.com>
Reviewed-by: Alex Vesker <valex@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>