xen-netback: remove page tracking facility

The data flow from DomU to DomU on the same host in current copying
scheme with tracking facility:

       copy
DomU --------> Dom0          DomU
 |                            ^
 |____________________________|
             copy

The page in Dom0 is a page with valid MFN. So we can always copy from
page Dom0, thus removing the need for a tracking facility.

       copy           copy
DomU --------> Dom0 -------> DomU

Simple iperf test shows no performance regression (obviously we copy
twice either way):

  W/  tracking: ~5.3Gb/s
  W/o tracking: ~5.4Gb/s

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Matt Wilson <msw@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>