Add the ndo_busy_poll handler. Use skb_mark_napi_id to mark receive
packets with the napi id. For each slice, use per-slice spinlock and
state variable to ensure that only one handler processes receive
pacekts.

Locking, statistics counters, and the ndo_busy_poll handler follow
those in the ixgbe driver. So, credit goes to that driver's authors.
Signed-off-by: Hyong-Youb Kim <hykim@myri.com>
Signed-off-by: David S. Miller <davem@davemloft.net>