xen-netback: refactor guest rx

xen-netback: refactor guest rx

Refactor the to-guest (rx) path to:

1. Push responses for completed skbs earlier, reducing latency.

2. Reduce the per-queue memory overhead by greatly reducing the
   maximum number of grant copy ops in each hypercall (from 4352 to
   64).  Each struct xenvif_queue is now only 44 kB instead of 220 kB.

3. Make the code more maintainable.

Signed-off-by: David Vrabel <[email protected]>
[re-based]
Signed-off-by: Paul Durrant <[email protected]>
Signed-off-by: David S. Miller <[email protected]>