net: provide generic busy polling to all NAPI drivers

NAPI drivers no longer need to observe a particular protocol
to benefit from busy polling (CONFIG_NET_RX_BUSY_POLL=y)

napi_hash_add() and napi_hash_del() are automatically called
from core networking stack, respectively from
netif_napi_add() and netif_napi_del()

This patch depends on free_netdev() and netif_napi_del() being
called from process context, which seems to be the norm.

Drivers might still prefer to call napi_hash_del() on their
own, since they might combine all the rcu grace periods into
a single one, knowing their NAPI structures lifetime, while
core networking stack has no idea of a possible combining.

Once this patch proves to not bring serious regressions,
we will cleanup drivers to either remove napi_hash_del()
or provide appropriate rcu grace periods combining.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/myricom/myri10ge/myri10ge.c

@@ -3814,7 +3814,6 @@ static int myri10ge_alloc_slices(struct myri10ge_priv *mgp)
 		ss->dev = mgp->dev;
 		netif_napi_add(ss->dev, &ss->napi, myri10ge_poll,
 			       myri10ge_napi_weight);
-		napi_hash_add(&ss->napi);
 	}
 	return 0;
 abort: