xen-netback: simplify xenvif_rx_meta

xen-netback: simplify xenvif_rx_meta

The amount of metadata stored during xenvif_rx_action can be reduced
drastically when one realizes that:

- The id field in xen_netif_rx_response must be identical to its 'slot'
  in the shared ring (the protocol requires it).
- When the responses are built, the code knows which skb they refer to
  and hence can extract GSO metadata at that point.

As a result the size taken by the metadata array is reduced and many
lines of code can be removed.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>