net/mlx5: E-Switch, Use dynamic alloc for dest array

Use dynamic allocation for the dest array in preparation for
the next patch which increase MLX5_MAX_FLOW_FWD_VPORTS and
will cause stack allocation to be bigger than 1024 bytes.

Signed-off-by: Maor Dickman <maord@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>