netmap: refresh netmap_if address after each NIOCREGIF

netmap: refresh netmap_if address after each NIOCREGIF

With the introduction of netmap "partial opening" feature
netmap requires that we get a new NETMAP_IF pointer after
every `NIOCREGIF` registration. Because this allocates an
independent instance of `struct netmap_if`. If one
separately opens hw rings and sw rings he/she'll get two
`struct netmap_if`, one with the valid hw rings, and the other
with valid sw rings.

Because of that we get a new netmap_if pointer after each
NIOCREGIF.

Also removing netmap_if struct from NetmapDevice since
it's no more required.

Ticket #2855.