via-rhine: forbid holes in the receive descriptor ring.

Rationales:
- throttle work under memory pressure
- lower receive descriptor recycling latency for the network adapter
- lower the maintenance burden of uncommon paths

The patch is twofold:
- it fails early if the receive ring can't be completely initialized
  at dev->open() time
- it drops packets on the floor in the napi receive handler so as to
  keep the received ring full

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: David S. Miller <davem@davemloft.net>