net/mlx5e: TC, Refactor mlx5e_tc_add_flow_mod_hdr() to get flow attr

[ Upstream commit ff993167

 ]

In later commit we are going to instantiate multiple attr instances
for flow instead of single attr.
Make sure mlx5e_tc_add_flow_mod_hdr() use the correct attr and not flow->attr.

Signed-off-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Oz Shlomo <ozsh@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Stable-dep-of: 2951b2e1

 ("net/mlx5e: Always clear dest encap in neigh-update-del")
Signed-off-by: Sasha Levin <sashal@kernel.org>