net/mlx5: CT: Fix header-rewrite re-use for tupels

Tuple entries that don't have nat configured for them
which are added to the ct nat table will always create
a new modify header, as we don't check for possible
re-use on them. The same for tuples that have nat configured
for them but are added to ct table.

Fix the above by only avoiding wasteful re-use lookup
for actually natted entries in ct nat table.

Fixes: 7fac5c2e ("net/mlx5: CT: Avoid reusing modify header context for natted entries")
Signed-off-by: Paul Blakey <paulb@nvidia.com>
Reviewed-by: Ariel Levkovich <lariel@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>