net/mlx5: CT: Avoid reusing modify header context for natted entries

Currently the driver is designed to reuse header modify context entries.
Natted entries will always have a unique modify header, as such the
modify header hashtable lookup is introducing an overhead. When the
hashtable size exceeded 200k entries the tested insertion rate dropped
from ~10k entries/sec to ~300 entries/sec.

Don't use the re-use mechanism when creating modify headers
for natted tuples.

Signed-off-by: Paul Blakey <paulb@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>