net/mlx5: DR, Fix wrong allocation of modify hdr pattern

Fixing wrong calculation of the modify hdr pattern size,
where the previously calculated number would not be enough
to accommodate the required number of actions.

Fixes: da5d0027 ("net/mlx5: DR, Add cache for modify header pattern")
Signed-off-by: Yevgeny Kliteynik <kliteyn@nvidia.com>
Reviewed-by: Erez Shitrit <erezsh@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>