net/mlx5e: TC, Keep mod hdr actions after mod hdr alloc

When offloading TC NIC rule which has mod_hdr action, the
mod_hdr actions list is freed upon mod_hdr allocation.

In the new format of handling multi table actions and CT in
particular, the mod_hdr actions list is still relevant when
setting the pre and post rules and therefore, freeing the list
may cause adding rules which don't set the FTE_ID.

Therefore, the mod_hdr actions list needs to be kept for the
pre/post flows as well and should be left for these handler to
be freed.

Fixes: 8300f225

 ("net/mlx5e: Create new flow attr for multi table actions")
Signed-off-by: Ariel Levkovich <lariel@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>