net/mlx5e: TC, Hold sample_attr on stack instead of pointer

In later commit we are going to instantiate multiple attr instances
for flow instead of single attr.
Parsing TC sample allocates a new memory but there is no symmetric
cleanup in the infrastructure.
To avoid asymmetric alloc/free use sample_attr as part of the flow attr
and not allocated and held as a pointer.
This will avoid a cleanup leak when sample action is not on the first
attr.

Signed-off-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Oz Shlomo <ozsh@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>