xen-netback: make sure skb linear area covers checksum field

skb_partial_csum_set requires that the linear area of the skb covers the
checksum field. The checksum setup code in netback was only doing that
pullup in the case when the pseudo header checksum was being recalculated
though. This patch makes that pullup unconditional. (I pullup the whole
transport header just for simplicity; the requirement is only for the check
field but in the case of UDP this is the last field in the header and in the
case of TCP it's the last but one).

The lack of pullup manifested as failures running Microsoft HCK network
tests on a pair of Windows 8 VMs and it has been verified that this patch
fixes the problem.

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