xen/netback: add fraglist support for to-guest rx

This allows full 64K skbuffs (with 1500 mtu ethernet, composed of 45
fragments) to be handled by netback for to-guest rx.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
[re-based]
Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>