net: xen-netback: use API provided by xenbus module to map rings

The xenbus module provides xenbus_map_ring_valloc() and
xenbus_map_ring_vfree().  Use these to map the Tx and Rx ring pages
granted by the frontend.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>