xen-netback: Unconditionally set NETIF_F_RXCSUM

There is no mechanism to insist that a guest always generates a packet
with good checksum (at least for IPv4) so we must handle checksum
offloading from the guest and hence should set NETIF_F_RXCSUM.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>