[NET]: Do not grab device reference when scheduling a NAPI poll.

It is pointless, because everything that can make a device go away
will do a napi_disable() first.

The main impetus behind this is that now we can legally do a NAPI
completion in generic code like net_rx_action() which a following
changeset needs to do.  net_rx_action() can only perform actions
in NAPI centric ways, because there may be a one to many mapping
between NAPI contexts and network devices (SKY2 is one example).

We also want to get rid of this because it's an extra atomic in the
NAPI paths, and also because it is one of the last instances where the
NAPI interfaces care about net devices.

The one remaining netdev detail the NAPI stuff cares about is the
netif_running() check which will be killed off in a subsequent
changeset.

Signed-off-by: David S. Miller <davem@davemloft.net>