net/sched: act_api: avoid non-contiguous action array

In tcf_action_add, when putting the reference for the bound actions
it assigns NULLs to just created actions passing a non contiguous
array to tcf_action_put_many.
Refactor the code so the actions array is always contiguous.

Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>