usbnet: optimize usbnet_bh() to reduce CPU load

usbnet: optimize usbnet_bh() to reduce CPU load

The current source pushes skb into dev-done queue by calling
skb_dequeue_tail() and then pop it by skb_dequeue() to branch to
rx_cleanup state for freeing urb/skb in usbnet_bh(). It takes extra CPU
load, 2.21% (skb_queue_tail) as follows,

-   11.58%     0.26%  swapper          [k] usbnet_bh
   - 11.32% usbnet_bh
      - 6.43% skb_dequeue
           6.34% _raw_spin_unlock_irqrestore
      - 2.21% skb_queue_tail
           2.19% _raw_spin_unlock_irqrestore
      - 1.68% consume_skb
         - 0.97% kfree_skbmem
              0.80% kmem_cache_free
           0.53% skb_release_data

To reduce the extra CPU load use return values to call helper function
usb_free_skb() to free the resources instead of calling skb_queue_tail()
and skb_dequeue() for push and pop respectively.

-    7.87%     0.25%  swapper          [k] usbnet_bh
   - 7.62% usbnet_bh
      - 4.81% skb_dequeue
           4.74% _raw_spin_unlock_irqrestore
      - 1.75% consume_skb
         - 0.98% kfree_skbmem
              0.78% kmem_cache_free
           0.58% skb_release_data
        0.53% smsc95xx_rx_fixup

Signed-off-by: Leesoo Ahn <lsahn@ooseel.net>
Signed-off-by: David S. Miller <davem@davemloft.net>