net/mlx5e: TC, Refactor sample offload error flow

Refactor sample unoffload to be symmetric to sample offload.
Use the existing del_post_rule() to release the post rule.
Also mlx5e_tc_sample_unoffload() should not return post_rule
which is NULL when post actions are supported.
Sample offload works with this NULL because many places of the
code use IS_ERR() instead of IS_ERR_OR_NULL() to check rule is valid
and when rule is detected as sample offload the code is not using the
rule. Let's be persistent and avoid returning NULL anyway and return the
pre rule, like in CT case, which is not NULL.

Signed-off-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Chris Mi <cmi@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>