net/mlx5: DR, Use variably sized data structures for different actions

mlx5dr_action is a generally used data structure, and there is an
union for different types of actions in it. The size of mlx5dr_action
is about 72 bytes, but for those actions with fewer fields, most of
the allocated memory is wasted.
Remove this union, and mlx5dr_action becomes a generic action header.
Then actions are dynamically allocated with needed memory, the data
for each action is stored right after the header.

Signed-off-by: Jianbo Liu <jianbol@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>