nat: replace speculative buffer enqueue model

nat: replace speculative buffer enqueue model

Replace speculative buffer enqueue coding model with
vlib_get_buffers(...)/vlib_buffer_enqueue_to_next(...).

Type: improvement
Change-Id: I7dbfac2234a7bd754c599857eb1d5b601da5bc7c
Signed-off-by: Klement Sekera <ksekera@cisco.com>