net/sched: act_tunnel_key: add support for "don't fragment"

extend "act_tunnel_key" to allow specifying TUNNEL_DONT_FRAGMENT.

Suggested-by: Ilya Maximets <i.maximets@ovn.org>
Reviewed-by: Pedro Tammela <pctammela@mojatatu.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>