xen-netback: handle page straddling in xenvif_set_hash_mapping()

There's no guarantee that the mapping array doesn't cross a page
boundary. Use a second grant copy operation if necessary.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>